Remove use of virtual call from non-virtual benchmark

It was my hope that the C++ compiler would be smart enough to see that
it was using a vtkm::Sphere of a concrete type and would therefore know
the function to call without consulting the virtual table. However, this
turned out not to be the case (at least on CUDA). This change creates a
dummy version of vtkm::Sphere that has no virtual methods.
903 jobs for true-virtual-functions