Add a warning when lexicographic used with default args and 5+ objectives

[odow]

Closes #137

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.74%. Comparing base (0be8f2f) to head (980debf).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #138   +/-   ##
=======================================
  Coverage   98.73%   98.74%           
=======================================
  Files          12       12           
  Lines        1109     1113    +4     
=======================================
+ Hits         1095     1099    +4     
  Misses         14       14           

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