Removed class decorator and added SECountyField to handle model accessors for full county name and numerical codes.

Also renamed `FULL_COUNTY_NAMES` and `NUMERICAL_COUNTY_CODES` in `se_counties.py` into `NUMERICAL_COUNTY_CODE_CHOICES` and `FULL_COUNTY_NAME_CHOICES` respectively, as well as converted them both to tuple lists rather than dicts in order for them to be supplied as the `chocies` argument for the field.