Address TODOs in Swahili Demographic Cleaner

[as2388]

core_data_modules.cleaners.swahili.demographic_cleaners.py and its associated unit tests file both still have some TODOS which need addressing.

This includes questions over the return type of some of the functions and whether cleaners should work on single-digit numbers rather than just double-digit numbers.

[as2388]

Also need to look in core_data_modules.regex_utils.py now that clean_with_pattern moved to here due to #33