feature: Internal Mesh Generator with regions specified from table

[rrsettgast]

This PR allows the InternalMeshGenerator to use a TableFunction in order to specify regions. The approach is as follows, but the details are up for discussion:

• The internal mesh generator can use a table of integers that correspond to region indices to specify what cells are in what regions.
• The concept of CellBlock remains, and is still used to specify CellBlockSubRegion inside a Region.
• The TableFunction be specified at in either the ElementRegions xml block (which corresponds indirectly with the ElementRegionManager) or in the InternalMeshGenerator block.

Tasks:

• Implement mechanism to use TableFunction to specify regions.
• Determine location of the TableFunction name. ElementRegions vs InternalMeshGenerator. Implement the input.
• Error checking for the table. What if the extents of the TableFunction are insufficient for the mesh? etc...
• Unit tests
• Integrated tests