Consistently using "mark_as_superbuild" along side the declaration
of the associated option or cache entry facilitates maintenance by
helping ensure value are consistently passed to the inner build.

For example, it fixes the passing of these two options:

* iMSTK_ENABLE_CUDA_BACKEND: Introduced in 5e90bbce (ENH: Enable CUDA
  in CMake configuration)

* iMSTK_SWIG_PINNED_ARRAY: Introduced in c2b5429e (ENH: add pinned array;
  add cmake option to choose between marshal array and pinned array)