Enable changing policy used for library compiles

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.