Bringing back support for rectilinear input.

vtkExtractCTHPart in past support a vtkRectilinearGrid as input. Commit 323ad383
resulted in that ability being lost. Fixed that. Also fixing the defaults for
the newly added filter parameters to match the old version.

Change-Id: Ie163572ea34d89564feff57f40f4e58c099a0100
Merge topic 'fix_cth_part_test' into master

558b34a1 Bringing back support for rectilinear input.