Round narrows

[jatkinson1000]

Adds n_arrows attribute to rounds as inexpensive and useful to be exposed at the top level rather than calculated from passes.

[jatkinson1000]

@TomHall2020 FYI: added this little feature as it was desirable for something else I'm doing. May be useful to you.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 97.82%. Comparing base (470d14e) to head (64fcfc6).
Report is 3 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #111   +/-   ##
=======================================
  Coverage   97.81%   97.82%           
=======================================
  Files          30       30           
  Lines        1742     1747    +5     
=======================================
+ Hits         1704     1709    +5     
  Misses         38       38           

Files with missing lines	Coverage Δ
archeryutils/rounds.py	100.00% <100.00%> (ø)
archeryutils/tests/test_rounds.py	100.00% <100.00%> (ø)