Add effective mass example

[houpc]

No description provided.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 95.46%. Comparing base (cc34581) to head (ef95625).
Report is 8 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master      #37   +/-   ##
=======================================
  Coverage   95.45%   95.46%           
=======================================
  Files          10       10           
  Lines        1365     1366    +1     
=======================================
+ Hits         1303     1304    +1     
  Misses         62       62           

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