Merge branch 'knepley/feature-plex-plot' into 'main'

Plex+Viewer: PETSCVIEWERPYTHON and PyVista support for DMPLEX

See merge request petsc/petsc!7884

Plex+Viewer: Added Python and PyVista viewers
- Enable viewer filename interface for Python viewer
- Add -viewer_python_filename to set the output filename
- Add -dm_plex_transform_active_values to e

Plex+Viewer: Added Python and PyVista viewers
- Enable viewer filename interface for Python viewer
- Add -viewer_python_filename to set the output filename
- Add -dm_plex_transform_active_values to explicitly set values in the label
- Add -dm_plex_option_phases to control sequencing of setup from the command line

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