Updated geo mapping/aggregation utility

* added the functions zip_to_state_code, zip_to_state_id (and the convert_* versions), zip_to_msa and convert_zip_to_msa
* added two functions add_geocode and replace_geocode meant to consolidate the logic in the utility and reduce the code size by a factor of 5. These functions work along side with the rest of the deprecated functions and are meant to replace e.g. zip_to_msa(df, ...) with replace_geocode(df, "zip", "msa", ...).
* renamed functions that referred to fips or county interchangeably to consistently use fips, e.g. zip_to_county to zip_to_fips
* enforced the string type on all geocodes, with zero padding as necessary
* renamed instances of stcode to state_code for clarity
* removed non-JHU UID functions for JHU conversion
* updated tests to match

Bugfixes:

* Removed .0000, 9xxx in output mappings - fixes most of #254
* Puerto Rico deaths should now be reported - fixes #179
* Generally fixes #215

Author: krivard