Fix issues with ExtractCTHParts filter.

This addresses several issues with vtkExtractCTHPart filter by doing a refactor
that cleaned up some of the logic and implementation. Also added backwards
compatibility fixes for XML state files since the filters API was changed to
remove inconsistencies.

Addresses BUG #14396.

Change-Id: I3e6b66338351d972aaf487cae6e6d521a5f04ff3
Merge topic 'fix_extract_cth_filter' into next

1aa4398 Fix issues with ExtractCTHParts filter.