pyvista/pyvista: Bug Fixes and General Cleanup

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

• 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)

• 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)