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