add an operate method for abs

[nsajko]

Fixes #241

[nsajko]

There's no tests here because #242 isn't merged yet. After it's merged abs just has to be added to the tested unary operations.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (ac7a8a9) 89.65% compared to head (f520983) 89.66%.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #243   +/-   ##
=======================================
  Coverage   89.65%   89.66%           
=======================================
  Files          23       23           
  Lines        2156     2157    +1     
=======================================
+ Hits         1933     1934    +1     
  Misses        223      223           

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[odow]

Can be rebased onto master and a test added