In VTK-m we have a constant tension between minimizing the number of
types we have to compile for (to reduce compile times and library size)
and maximizing the number of types that our filters support.
Unfortunately, if you don't compile a filter for a specific array type
(value type and storage), trying to run that filter will simply fail.
To compromise between the two, added methods to `DataSet` and `Field`
that will automatically convert the data in the `Field` arrays to a type
that VTK-m will understand. Although this will cause an extra data copy,
it will at least prevent the program from failing, and thus make it more
feasible to reduce types.
The list defining the common storage types was defined in
DefaultTypes.h.in. The problem was that derived default types could not
just add their own type. They had to redefine the whole list.
Instead, move the list to StorageList.h. Also updated
DefaultTypesVTK.h.in to use this list when using XGC. This enables
ArrayHandleSOA for this case.
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.
