to address this issue:
alan-turing-institute/rctab-functions#42

tests are empty for now

 On branch tests_for_post_usage
 Changes to be committed:
	modified:   tests/test_routes/test_usage.py

rctab_models also updated to reflect the update class definition.
poetry dependency also updated to use a relative path to
rctab-models (which a local copy with the updated class definition)

logic and tests to delete existing rows within date range in
post monthly usage