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 :white_check_mark:

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

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Additional details and impacted files

```diff @@ 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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