Allow searching for CSVs at arbitrary depths

[bethac07]

Right now cytominer-database expects you'll point it at one folder, which has subfolders, which have your CSVs- it'd be nice to be able to make this more general- ie if I want to make a database from many plates, my CSV structure wouldn't match. Likely this could be fixed by passing an explicit depth keyword for where find_directories should search for the files but I'm agnostic as to what's the best solution.