Allow overlapping “main” patterns and subpatterns

[ISSOtm]

teNOR currently assumes that it can compute the "overlap" optimisation by comparing the cells themselves; this is not true since a913a51, because nothing guarantees a given cell has the same ID across both catalogs.

Therefore, cells should instead be compared via some kind of IDs, of three kinds: shared, main only, and subpattern only. Then, the shared cells would get the same indices across both catalogs (so the first few, for simplicity).