Initial support for HalfGrid and ComputeType

[ghurstunither]

Initial support for HalfGrid and ComputeType in OpenVDB, where the latter enables intermediate float computations for HalfGrid to increase accuracy and speed.

• Uses ValueToComputeMap in Types.h to set ComputeType at the leaf level and then to propagate all the way up to the grid level.
  • For math::half, ComputeType is float and for any other type T, ComputeType is T.
• Defines and registers HalfGrid (half vectors not in this PR).
• Moved the inclusion/declaration of math::half from Types.h to its own HalfDecl.h
  • Added some arithmetic support for math::half in Math.h (to avoid casting in some places) so there were now circular dependancies.
• Refactors many tools and utilities to perform intermediate computations with ComputeType instead of ValueType.

Note, I decided not to define openvdb::is_floating_point and friends. Instead I opted to use std::is_floating_point<ComputeType>()::value instead.

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Still to come in this PR

*Uunit tests for half and to verify no unit tests have broken.

• Quantification of the level of support in tools.
  • e.g. some tools didn't need refactoring, and so this PR doesn't reflect that they fully support half grids
• More examples within this description.