[BUG] Vec3H Grid Registration no longer builds on ToT master

[matthewdcong]

To reproduce, add the following registration to vdb_print/main.cc: openvdb::Grid<openvdb::tree::Tree4<openvdb::Vec3H, 4, 3, 3>::Type>::registerGrid();

This will fail to build with a no match for operator+ error on GCC 7.3.1 but building with Clang 16 reveals more information:

openvdb/openvdb/math/Vec3.h:540:43: note: candidate template ignored: requirement 'std::is_arithmetic_v<openvdb::v11_0::math::internal::half>' was not satisfied [with S = float, T = openvdb::v11_0::math::internal::half]
inline Vec3<typename promote<S, T>::type> operator+(const Vec3<T> &v, S scalar)

This appears to be due to #1688 because of the enable_if because std::is_arithmetic is true for a hardcoded list of system types that does not include half.

I've attempted to come up with a fix, but I've ran into two issues so far. Specializing std::is_arithmetic is undefined behavior, so we'll need to define an alternative template check. Secondly, this would create a circular dependency of sorts between Math.h and Types.h, since the latter is where the half type, either internal to OpenVDB or externally imported from Imath, is defined.