Include outputs when writing input file

[christophe-david]

This PR adds an optional feature, in API and CLI, to include output variables when writing the input file.

It is done "roughly": all output variables are written, without considering if a variable is part of a cycle or not.

[codecov]

Codecov Report

Attention: Patch coverage is 95.23810% with 1 line in your changes missing coverage. Please review.

Project coverage is 85.36%. Comparing base (c0143b7) to head (4dbb289).

Files	Patch %	Lines
src/fastoad/cmd/api.py	66.66%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #552      +/-   ##
==========================================
+ Coverage   85.32%   85.36%   +0.03%     
==========================================
  Files         138      138              
  Lines        6414     6422       +8     
  Branches      940      943       +3     
==========================================
+ Hits         5473     5482       +9     
+ Misses        785      784       -1     
  Partials      156      156              

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