Radial eigenfunction fix

[Y-Bimali]

Fixes bug in R_1n eigenfunction for the nmk=0 case. It also changes the references to the scale function to distinguish it and the outer radius when necessary, but this shouldn't change code behavior or results in our use cases so far.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.38710%. Comparing base (fb0bb34) to head (431c74a).
⚠️ Report is 4 commits behind head on main.

Additional details and impacted files

@@                 Coverage Diff                 @@
##                main         #76         +/-   ##
===================================================
- Coverage   98.40183%   98.38710%   -0.01473%     
===================================================
  Files             11          11                 
  Lines           1314        1302         -12     
===================================================
- Hits            1293        1281         -12     
  Misses            21          21                 

Flag	Coverage Δ
unit	98.38710% <100.00000%> (-0.01473%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.