BUG: Fix find_input() Function to Return a Single Value

[MateusStano]

Pull request type

• Code changes (bugfix, features)
• Code maintenance (refactoring, formatting, tests)
• ReadMe, Docs and GitHub updates
• Other (please describe):

Checklist

• Tests for the changes have been added (if needed)
• Docs have been reviewed and added / updated
• Lint (black rocketpy/ tests/) has passed locally
• All tests (pytest --runslow) have passed locally

New behavior

Scipy's optimize.root gives an array of length 1 to the Function object inside find_input. Currently if a Function receives an Iterable, it returns a list. This was not always the case before #451.

Now find_input should always work, independent of the Function source

[codecov]

Codecov Report

Attention: 2 lines in your changes are missing coverage. Please review.

Comparison is base (5cdb31a) 70.86% compared to head (2e9a167) 70.86%.
Report is 26 commits behind head on develop.

Files	Patch %	Lines
rocketpy/mathutils/function.py	60.00%	2 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff            @@
##           develop     #471   +/-   ##
========================================
  Coverage    70.86%   70.86%           
========================================
  Files           55       55           
  Lines         9227     9227           
========================================
  Hits          6539     6539           
  Misses        2688     2688           

Flag	Coverage Δ
unittests	70.86% <60.00%> (ø)

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[Gui-FernandesBR]

It passes on my --runslow too (linux)