Merge master into next

* commit 'aa525d5d662ea6c7036e47a07e4c8a146a773e5f':

Change-Id: I35b38b1b4cd7cf7a45ce16b76693f781c3fe5785
Merge topic '13726_fix_variable_selection_performance' into next

d428305 BUG #13726. Fixing performance issues with updating variables.
Merge topic '13739_fix_variable_selection' into next

36c9d88 BUG #13739. Fixes issues when user clicked on the check-box itself.
Merge topic '13769_contour_polydata_scalars' into next

8ba7780 Enabling scalars for polydata contouring.
Merge topic 'gmv_fixes' into next

ead3c7b ENH: Reworked the tests for GMVReader plugin
f83f39f ENH: Raise warnings when input file contains more than 1000 materials