CMAKE: add unity builds support
Very experimental, it only works with with recent CMAKES, older CMAKE will just ignore this.
It allows grouping instantiations files through the cmake variable -DCMAKE_UNITY_BUILD_BATCH_SIZE=X setting default batch size to 1, since this is the safest choice.
Here is a benchmark exploring the performance using different batch sizes:
https://gitlab.kitware.com/vbolea/vtkm-benchmarks/-/tree/master/10-12-21_instantiations