Add 2nd x-axis for plots with inverse microns as the main x axis

[karllark]

Closes #112

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.89%. Comparing base (6601d6a) to head (fade962).
Report is 26 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #230   +/-   ##
=======================================
  Coverage   99.89%   99.89%           
=======================================
  Files           8        8           
  Lines         935      935           
=======================================
  Hits          934      934           
  Misses          1        1           

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