Test that triggers the len error

[gordonwatts]

• Added len as an "internal" function, much the way abs is already added.
• Added small test to make sure this fixes the warning we've been seeing.

Fixes #146

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 96.94%. Comparing base (29a3406) to head (b51dae6).

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #151   +/-   ##
=======================================
  Coverage   96.93%   96.94%           
=======================================
  Files          15       15           
  Lines        1338     1340    +2     
=======================================
+ Hits         1297     1299    +2     
  Misses         41       41           

Flag	Coverage Δ
unittests	96.94% <100.00%> (+<0.01%)	⬆️

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