More detailed defaults for problem.get_full_vector

[PaulJonasJost]

As the doctoring already explained before, we usually want problem.x_fixed_vals as defaults if we update a vector x. I think this should be a default and leaving the nans should be left for gradients (hence another parameter to distinguish the two.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 84.52%. Comparing base (18be6e3) to head (c97d36c).

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Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #1393      +/-   ##
===========================================
+ Coverage    84.50%   84.52%   +0.02%     
===========================================
  Files          157      157              
  Lines        12955    12958       +3     
===========================================
+ Hits         10947    10953       +6     
+ Misses        2008     2005       -3     

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[Doresic]

Looks good.
Makes more sense for the default to grab problem.x_fixed_vals instead of not using them.

[m-philipps]

Thank you! I left an optional suggestion to extend on the docstring.