Pyvista

This update brings some major changes to the widgets (the previous API for widgets is broken. Please see new API)

New Features

- New Sphere and Spline widgets (see 383 and 401) with major overahul to allow support from many widgets in a single scene
- New OBJ scene exporter (see 392)
- New toolbar for the `BackgroundPlotter` for controlling the camera (see 351)
- `UniformGrid` objects can now be cast to `RectilinearGrid` objects (see 381)

Bug Fixes

- NaN camera position issue resolved (see 383)
- Scalar mapping issues for `uint8` arrays fixed (see 384)
- Fix `q`-key for closing the `BackgroundPlotter` (see 388)
- Properly grab screenshots when `BackgroundPlotter` closes (see 382)
- Fix issue when trying to plot multiple sets of point labels
- Add access to repeat option on the `Texture` class
- new viewport parameter to control text location in `BasePlotter.add_text` (see 390)
- Resolve some major Python 2.7 issues for backwards compatibility (see 393)
- Ignore strange errors when automatically grabbing depth image
- Fix issue with `lines_from_points` (see 394 and 399)
- Fix issue where `Plotter` class couldn’t accept the `title` argument.

New Features

- New depth mapping feature for the rendering window (see 369 and 368)
- New layout options for subplotting (see 380). Many thanks to marcomusy
- Anti-aliasing features (see 365)
- Background color and number of colors are now settable for the scalar bar (see 370)
- New relaxation parameter for the `PolyData.smooth` filter (see 374)
- Major overhaul of the widget tools to improve performance (see 376)
- New `shallow_copy` and `deep_copy` methods (see 377)
- Improve default options for texture mapping
- Wrapped the `vtkTexture` class
- Enable use of `edl` keyword argument for eye dome lighting in the `.plot()` function.
- New shortcuts for the `point_data_to_cell_data()` and `cell_data_to_point_data()` filters as `ptc()` and `ctp()` respectively.

Bug Fixes

- Minor improvements for supporting Panel (ref 268)
- Fix `BackgroundPlotter` menu bar on some Linux platforms (see 371)
- Much cleanup, typos, etc., fixed by marcomusy in 380
- Much PEP8 cleanup inside the library

There was a serious bug with the plane widget that somehow made it in the last release. This patch release resolves that issue and adds support for `MultiBlock` datasets with the widgets.

New Features

- Add full support for widgets in the rendering scene! (See 360)
- Box widget - Create a box in 3D space. Includes a convenience method for clipping meshes
- Plane widget - Create an implicit plane in 3D space. Includes a convenience method for clipping and slicing meshes
- Slider widget - Add a slider bar to the render window. Includes a convenience method for thresholding meshes
- Line widget - Create and orient a line in 3D space.
- Point picking (see 362)
- Single point picking on meshes and in 3D space
- Trace lines across meshes using a geodesic path
- Trace lines through a scene by picking points on meshes or in 3D space
- Trace out horizon surfaces
- Wrap the `vtkTable` class (see 274 and the new `pyvista.Table` class)
- Add a structured grid cylinder helper method (see `pyvista.CylinderStructured`) which works creates a `pyvista.StructuredGrid` mesh in the shape of a cylinder.
- New `.ribbon()` filter for `PolyData` objects - create surfaces (“ribbons”) from lines
- `interpolate_befor_map` is now set too `True` as default (see 363 and [this example](https://docs.pyvista.org/examples/02-plot/interpolate-before-map.html) for details)


Bug Fixes

- Resolve issues with the `lines_from_points` helper methods (see 357)
- Fix an issue when fetching null points on datasets

Exciting New Features

- Labeled scalar bars!!! 🎉 (See 348 and [this example](https://docs.pyvista.org/examples/02-plot/scalar-bars.html)) Now arrays with strings categories can be color mapped
- Ability to clip a mesh using the surface of another mesh (see 354 and [this example](https://docs.pyvista.org/examples/01-filter/clipping.html#clip-with-surface))
- Parallel projection (see 337)
- New MCubes reader (`.tri` files)
- New cell quality and gradient filters (see 354)
- Implement the `+` operator to support all PyVista meshes


Bug Fixes
- Subplotting borders fixed
- Handle arrays with no name (see 306)
- Fix the `fly_to` method
- Typos and grammar fixed in the example gallery - huge thanks to JasonTlin
- Fixed issue with the `+` operator that appeared in `v0.21.3`
- Clear the menu bar for the BackgroundPlotter when closing on Mac OS (see 350)

This release is primarily full of bug fixes and general cleanup around the filters and plotting methods.

Bug Fixes

- Fix points ordering issue with gridded datasets
- Fix issues with overlapping colorers (see 319)
- Fix rare bug when scalars are the wrong size
- Handle `panel` runtime errors on import (see 322)
- Fix `smooth_shading` issue for non-`PolyData` datasets
- Fix subplot indexing issues (see 346)


Changes

- Memory footprint is improved (see 338)
- `panel` notebook rendering is now off by default
- New downloadable examples!
- Filters were overhauled into their own module and generalized across datasets (see 323)
- Improved control over axes widgets (see 334 and 342)
- Improvements to `add_mesh` and `add_volume` docstrings
- Deprecate `BasePlotter.plot` (see 341)
- Enable use of poly lines with `PolyData` (see 344)
- Add more lighting controls (see 343)