DIY now depends on MPI optionally. Hence we no longer need to depend on DIY optionally based on whether MPI was enabled. Update cmake and c++ code to always use DIY-based components. DIY is built with MPI support if VTKm_ENABLE_MPI is ON.
VTKM_DIY()
This makes it easier to mangle diy and support using external diy.
EnvironmentTracker
When using MPI, we need a mechanism in VTK-m to set/get the MPI controller. Added `EnvironmentTracker` to do that.