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

In cytomining/pycytominer#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.