use interface library for vtkm dependencies

In CMake/vtkm.cmake define an interface library that encapsulates our usage of VTKm. Make sure that SENSEICOnfig includes the exported libnrary so that external projects can link to sensei built with VTKm features.

see for example CMake/vtk.cmake, CMake/mpi.cmake, CMake/adios.cmake for examples of how this is done.