Deprecate ArrayHandleVirtual

Virtual methods in the execution environment are going away the next major release of VTK-m, so deprecate their use in preparation for that.

A CMake option named VTKm_NO_DEPRECATED_VIRTUAL is added. It defaults to OFF. But when it is ON, none of the code that both uses virtuals and is deprecated will be built.

Currently, only ArrayHandleVirtual is deprecated, so the rest of the virtual classes will still be built. As we move forward, more will be removed until all virtual method functionality is removed.