MPI GPU interface refactoring

[ethanglaser]

Description

Changes proposed in this pull request:

• Add virtual get_mpi_offload_support function to base communicator - defaults to false in nearly all cases
• Add logic to get_mpi_offload_support function in mpi/communicator.h to check mpi libs for correct symbol and determine if level zero is supported
• Add conditional in detail/communicator.cpp that uses result of get_mpi_offload_support to determine whether to convert data to host (previous default) or leave as is (yields performance improvements if GPU offload support in MPI)
• Modify sendrecv_replace args to include optional additional buffer to accommodate MPICH workaround to call sendrecv with 2 GPU buffers