Add in the vtkm namespace an assert macro (technically VTKM_ASSERT) that
basically replicates the functionality of the POSIX assert macro. This
form of assert is set to replace the separate control/exection asserts.

It has been decided that an assert that throws an exception instead of
terminating the program is not all that great of a feature and it causes
some limitations on how it is used. The next commit will remove the
other forms of VTK-m assert.