Refactor/split remap_via_sub_cells() adding intersect_src_tgt_grids()

- The first block of remap_via_sub_cells() computes the intersection between
  two columns (grids). I've split out this block into a stand alone function
  so that we can re-use it in a to-be-written analog of remap_via_sub_cells()
  that will remap integrated quantities.
- Added some in-line documentation of algorithm used by remap_via_sub_cells()
- Added new column-wise function intersect_src_tgt_grids()
- Added tests of intersect_src_tgt_grids() that check against known results
  - Had to add a new helper function to MOM_remapping to report state of
    integer arrays.