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.

[jatkinson1000]

Debated on this a little as it doesn't match the new setups, but given the differences (an this being somewhat legacy/separate) I think it makes sense. Done both in 215c852

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 97.82%. Comparing base (ad97498) to head (215c852).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #105   +/-   ##
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  Coverage   97.82%   97.82%           
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  Files          30       30           
  Lines        1747     1748    +1     
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+ Hits         1709     1710    +1     
  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)