Allow a dual to be raised to a complex power.

[jwscook]

This PR adds a feature to enable duals to be raised to complex powers.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 52.58%. Comparing base (d9251a7) to head (3f3df91).
⚠️ Report is 2 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #102      +/-   ##
==========================================
+ Coverage   50.94%   52.58%   +1.63%     
==========================================
  Files           2        2              
  Lines         212      213       +1     
==========================================
+ Hits          108      112       +4     
+ Misses        104      101       -3     

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