Detect compartments in the single cell input file and throw a warning if they are not used

[gwaybio]

In #111 - I add a method in the cells.py class to merge tables using predefined "linking" columns.

If there are more than the default compartments in the provided input file (e.g. single cell SQLite), then the merge_single_cells() method will silently ignore the additional compartments if the method is not used properly.

We should throw a warning if there is a missing table not used.