Pyvista

- Fix 32bit Python on 64bit OS ID type issues (see 316)
- `BackgroundPlotter` changes:
- Window title (see 305)
- Add callback support (see 304)
- Fix closing issues (see 315)
- Add offscreen support (see 309)
- Fix window resizing issue (see 315)
- Fix typos
- Add `null_value` support for the `.interpolate` filter
- Replace AppVeyor with Azure Pipelines for Windows testing (see 317)
- Add more special array names to the exclusion list for active scalars (see 307)
- Update docstring for `.add_text` to detail how return type can vary (see 312)

This release is loaded with bug fixes and enhancements that weren't quite ready at `v0.21.0`

New Features

- Added option to use a scalar array for opacity mapping or custom transfer functions (see 297)
- Overhauled the `.clip` filter to no longer triangulate regions unaffected by the clipping plane (see
https://github.com/pyvista/pyvista-support/issues/13 and [this discussion](https://discourse.vtk.org/t/is-it-possible-to-turn-off-triangles-on-vtkclipdataset-filter/279))
- Add surface cell picking for visible cells only (see 281 and 277)
- Use Scooby’s `.Report()` convention (see 284)
- Improved point labels behavior and formatting (see 286)



Bug Fixes

- Changed defaults for volume rendering to make sure it typically works on all OS and all GPUs
- Fixed non-uniform subsetting issue with the `.glyph` filter (see 302)
- Fixed installation issue on CentOS
- Point labels can now be tracked by name when plotting
- Fixed bug where the cell picking callback would be called after empty selection
- Fixed issue where `MultiBlock` datasets weren’t wrapping properly
- Fixed volume rendering of `MultiBlock` datasets
- Fixed camera clipping planes bug when orbiting a scene
- Fixed issue if passing `texture=False` to `add_mesh`
- Fixed coloring issue with Box axes widget
- Fixed issue when shading a dataset and passing an array by name
- Fixed issue where `parse_color` would get tricked by RGBA tuples
- Fixed `BackgroundPlotter` closing event bug (see 293)
- Added default window title to `rcParams`

New Features

Most Exciting

- Volume Rendering (see 272 and 231) 🎉 🎆 - many thanks to supersubscript!
- Show selection interactively with `enable_cell_picking` (see 278)
- Slice along line filter (see 267)
- Default colormap is now `viridis`

Also Pretty Exciting

- Compute arc lengths of poly lines and splines (see 252)
- Enable subset glyphing (see 269)
- Enable filters on composite datasets (see 270)
- Major reorganization of internal modules to clean up the API (see 272)
- `PolyData.project_points_to_plane` filter (see 276)
- Nee dependency: [`scooby`](https://github.com/banesullivan/scooby) for `pyvista.generate_report()` to make bug reporting easier.
- Support for [`colorcet`](http://colorcet.pyviz.org) and [`cmocean`](https://matplotlib.org/cmocean/) colormaps

Bug Fixes

- Fix color mapping issue for uint8 arrays (see 266)
- Fix bugs with the auto documentation
- Fix `create_grid` if cell dimension is `1`
- Use a better default for `add_text` so that text stays in the corner (see 275)
- Fix frog example (see 237)
- Enhance clipping algorithm for `UniformGrid`s

This release has quite a few significant bug fixes and some useful new features inspired by user requests and contributions

New Features

- New linked cameras feature for subletting (see 255)
- Add new `plot_over_line` filter (see 250)
- Clearing scalar arrays (see 232)
- Add support for Splines (see 246)
- More parametric geometries
- Add method to clear textures: `.clear_textures()`
- New internal class, `PointSet` to manage common attributes between point based datasets

Bug Fixes

- Fix the frog example (see 237)
- Fix bug with `pyvista.Cone` (see 249)
- Fix bug where `PolyData` constructor did not always produce vertex cells (see 227)
- Fix length calculation for geodesic path (see 261)
- Other bug fixes with respect to the geodesic path filter

- New Supertorid geometric object (see 225 and 242)
- New software contract with Sphinx-Gallery to make Using PyVista with SG easier (see https://github.com/sphinx-gallery/sphinx-gallery/pull/494)
- Improved smooth shading (see 230)
- Add camera position argument for `.show()` method
- Update orbiting function
- Improve output of `.clip` filter (see 238)
- Improve error message for reading invalid files (see 239)

This marks the release of a lot of new features since about `v0.18.0`. We've
been under rapid development lately and these features are also comming with
our review in the [Journal of Open-Source Software](http://joss.theoj.org)
(JOSS). Listed below are some notable changes.

Enhancements

- Updates to `pyvista.rcParams`
- Add lots more example data download functions (see 195)
- PolyData filters updates (see 194)
- Add `.streamlines()` filter (see 199)
- Add `.interpolate()` filter (see d0baf2c)
- Refactor keyword argument `scale_factor` to `factor` (see 367e5bb)
- Add support for string scalar arrays and better point labelling (see 200)
- Add `MultiBlock.copy()` method (see 1481696)
- Add `MultiBlock` negative indexing support (see 1481696)
- Enable default plotting theme environment variable (see 3aeb812)
- Add `PolyData.extract_selection_points()` filter (see 205)
- Add support for field data arrays (207)
- Enhance `__repr__` methods (see a86c3ff)
- Add box orientation axes widget (see 218)


Bugfixes

- Fix window closing bug on Windows (see 190)
- Fix SEGY reader import issue (see 8f52ddb)
- Fix broken file not found error message (see 5ccad7a)
- Fix scalar bar slot issue in multi plots (see 6235f57)
- RGB plotting bug-fix (see cf4e6cd)
- Fix plot texture example (216)