For some reason when VTK-m was being compiled as an accelerator in VTK,
Visual Studio 2013 gave a bunch of warnings about not being able to generate
assignment operators for many classes. This happened for classes with a
const ivar that could not be automatically set. (Automatic copy constructors
are fine on this count.) I'm not sure why these warnings did not happen
when just compiling VTK-m, nor am I sure why they were generated at all as
no code actually used the copy constructors.

This commit fixes the problems by adding a private declaration for assignment
operators that cannot be automatically created. No implementation is
provided, nor should any be needed.