Previously, the PolicyDefault used to compile all the filters was hard-
coded. The problem was that if any external project that depends on VTK-
m needs a different policy, it had to recompile everything in its own
translation units with a custom policy.
This change allows an external project provide a simple header file that
changes the type lists used in the default policy. That allows VTK-m to
compile the filters exactly as specified by the external project.
