Fixes #216

Also if .cu files are passed as sources to a unit tests we
don't shim them

Due to limitations in the CUDA MSBuild support and how CMake stores the language
of a source file, we had to change VTK-m over to using generated .cu files
to signal when we want CUDA compilation.

Previous versions of the test build infrastructure would cause cmake to re-run
everytime a header was modified. Now this will not occur.

This is done to be more consistent with the vtkm CMake naming and
to make sure we don't conflict with other exported diy instances.

Previously all the Unit_tests that shared source files would use cuda
once CUDA set the language on the file.

there appears to be a bug in the naming of a source dir for VTKmExportHeaderTemplate.h.in so it can't find the config file

Renamed vtkmBackends.cmake to vtkmDeviceAdapters.cmake and renamed all references to vtkmBackends.cmake to use the new naming scheme.

Obviously this does mean that CUDA is not supported with 3.3.