Add generalized instantiation

Recently, an instantiation method was added to the VTK-m configuration files to set up a set of source files that compile instances of a template. This allows the template instances to be compiled exactly once in separate build files.

However, the implementation made the assumption that the instantiations were happening for VTK-m filters. Now that the VTK-m filters are being redesigned, this assumption is broken.

Thus, the instantiation code has been redesigned to be more general. It can now be applied to code within the new filter structure. It can also be applied anywhere else in the VTK-m source code.