Merge topic 'cleanup_cellset_explicit'

8bc40880 Add a test for CellSetExplicit::GetIndices
fa81d1de CellSetExplicit always calls methods using "this->"
9e496306 Allow incremental construction of CellSetSingleType.
3e307879 Mark CellSetExplicit::Fill as host side only.

Acked-by: Kitware Robot <kwrobot@kitware.com>
Merge-request: !227