Last refactor of remap_via_sub_cells() adding remap_sub_to_tgt_grid()

- Added remap_sub_to_tgt_grid() to handle remapping of intermediate
  state on sub-layers to the target grid. This function will be reused
  by alternate remap_via_subcells() analogs for extensive quantities.
- Added unit tests for remap_sub_to_tgt_grid().