vtkReflection: add variant conversion to float
pvpython on macos fails with:
./pvpython
dyld[9224]: symbol not found in flat namespace (__ZN3vtk12variant_castIfEET_RK10vtkVariant)
i.e.
dyld[9224]: symbol not found in flat namespace float vtk::variant_cast<float>(vtkVariant const&)
this commit should fix the issue