Reclaim some of the size lost by splitting the cell catalogs

[ISSOtm]

There is already somewhat considerable padding within a cell catalog, and splitting the catalog in two in https://github.com/ISSOtm/fortISSimO/commit/a913a51253111184e37d38847800bbcda01aac47 has multiplied the padding by 2.5!

This can be mitigated by fusing the two catalogs like they were before a913a512 when possible; then both labels are identical, and only a single header byte is wasted. However, that increases teNOR's complexity significantly.