support more types in DataSetWriter

DataSetWriter is fairly hardcoded to require specific types, so this update fixes that, at least for unstructured grids (also in the case of CellSetSingleType). This was tested with datasets created by VTK's VTK to VTK-m conversion, so it's possible that there could still be some situations that would fail, but I've put in errors so that should be easy to figure out.

This also supports more types when writing Fields.