Change the HyperTreeGrid mechanisms to provide efficient storage of very
large level 0 grids with only few cells defined. Instead of creating a
full grid array, only defined cells (through MaterialMaskIndex array)
are stored and processed.
The source has been updated to take advantage of this new feature.

Some optimizations and code simplification are also provided.

Provide a new test TestHyperTreeGridTernary3DGeometryLargeMaterialBits
to test the large grids management.

Set 3: Fix tests, mainly fix CopyStructure.

Change-Id: Iaab0afd7d13e66d9ab105c554a4d873ab59f1d35