Openpmd-viewer

This release adds two features:
- Improved calculation of the laser envelope, using the Hilbert transform.
- Reconstruction of full 3D field from a quasi-3D dataset, when passing `theta=None`.

This is a bug-fix release. It allows the slider to work properly in JupyterLab,
by using the `%matplotlib widget` magic.

This version includes minor improvements to the viewer:
- (Experiemental) support for Windows users
- In the interactive Jupyter GUI, the user can now select the scale of the vertical axis.
- The function `get_emittance` has more options (including calculation of the slice emttance)
- The default `openPMD_notebook` now avoids warning messages about matplotlib inline, which used to occur even though `%matplotlib notebook` was used.

Many thanks to MaxThevenet and AngelFP for their contributions to this release!

This version introduces several improvements to the viewer:
- The ability to read files that contain fields in different geometries
(e.g. 3D fields and 2D slices).
- Better support for files that do not contain mesh (or do not contain
particles), including support for the openPMD 1.1.0 standard.
- Cloud-In-Cell deposition in histograms.
- Better handling of `%matplotlib notebook` for newer version of jupyter.

This version adds better support, when the local installation of matplotlib
has issues:

- The `LpaDiagnostics` can now work without matplotlib if needed.
- The `MacOSX` matplotlib backend is now avoided, since there can be issues
when using it in the latest version of Jupyter.

This version improves support for `ipywidgets` version 7, especially in
the layout of the slider.

In addition, with this version of `openPMD-viewer`, `matplotlib` is not a
strict requirement anymore. This allows lighter installation for users that
need `openPMD-viewer` only as a data reader.

Finally, the calculation of the laser envelope in 2D has been improved
(see [PR 170](https://github.com/openPMD/openPMD-viewer/pull/170)). Note
that the function `wstd` (which is not documented in the tutorial, but
which some users might still use) has been renamed to `w_std`.