tweaks to linelist pruning

[ajwheeler]

• use reduced EW instead of EW, integrate between +- 90 km/s, not 2 A. (Thanks to T. Nordlander for suggestion.)
• return wavelength extrema of each multiplet
• clarify that merged lines are wl-sorted, and test that.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 90.73%. Comparing base (1105e8c) to head (c07cb97).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #284      +/-   ##
==========================================
+ Coverage   89.95%   90.73%   +0.77%     
==========================================
  Files          27       27              
  Lines        2130     2136       +6     
==========================================
+ Hits         1916     1938      +22     
+ Misses        214      198      -16     

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