Handle distance and diameter units in a better way

[jatkinson1000]

Closes #53

As raised by @TomHall2020 in #52 this simplifies and improves the method for setting units for distances and diameters.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (133abdb) 95.38% compared to head (0ecd135) 95.42%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #54      +/-   ##
==========================================
+ Coverage   95.38%   95.42%   +0.03%     
==========================================
  Files          23       23              
  Lines        1301     1311      +10     
==========================================
+ Hits         1241     1251      +10     
  Misses         60       60              

Files	Coverage Δ
archeryutils/__init__.py	100.00% <ø> (ø)
archeryutils/classifications/__init__.py	100.00% <ø> (ø)
...utils/classifications/agb_field_classifications.py	98.50% <ø> (ø)
...tils/classifications/agb_indoor_classifications.py	98.43% <ø> (ø)
.../classifications/agb_old_indoor_classifications.py	97.91% <ø> (ø)
...ils/classifications/agb_outdoor_classifications.py	99.15% <100.00%> (ø)
...cheryutils/classifications/classification_utils.py	90.38% <ø> (ø)
...cheryutils/classifications/tests/test_agb_field.py	100.00% <ø> (ø)
...heryutils/classifications/tests/test_agb_indoor.py	100.00% <ø> (ø)
...utils/classifications/tests/test_agb_old_indoor.py	100.00% <ø> (ø)
... and 11 more