Return codes for old field classifications

[jatkinson1000]

Fixes #104

Replaces the return array values with codes instead ['GMB', 'MB', 'B', '1C', '2C', '3C', 'UC'], and any unclassified by 'UC'.

Tests also updated to match new return values.

[jatkinson1000]

@TomHall2020 how do you feel about this?

[TomHall2020]

Ooh, nice. Gives me a reference to use for the codes to use for the old outdoor system too, and I definitely like 1C for first class, for some reason I coudln't think of that when I was trying to shorten the codes.

Wondered about assigning a global constant UNCLASSIFIED="UC" for DRY purposes, and could also think about making the list of codes a module level constant as well, could be handy for introspection, but those are seasoning. Rest is simple enough, reccomend merging it in.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 97.81%. Comparing base (e0d8d00) to head (d56b69b).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #105   +/-   ##
=======================================
  Coverage   97.81%   97.81%           
=======================================
  Files          30       30           
  Lines        1742     1742           
=======================================
  Hits         1704     1704           
  Misses         38       38           

Files with missing lines	Coverage Δ
...s/classifications/agb_old_field_classifications.py	100.00% <100.00%> (ø)
...yutils/classifications/tests/test_agb_old_field.py	100.00% <ø> (ø)