Datafile enhancement

[christophe-david]

This PR aims at easing a bit the usage of DataFile class:

• fixes #421 and #429
• allows instantiating a DataFile with no argument or from list of a Variable instances
• adds the save_as() method
• adds a documentation section about usage of DataFile (preview here)

BTW, I changed a bit the GitHub workflows to take advantage of the cache management of the setup-python action, that works for Poetry (bye bye actions/cache). I used this branch as a sandbox to test it.

Finally, since CodeClimate was nagging me with a "two many nested control flows" in the dreaded VariableList.from_problem() method (just because I slightly modified it to fix issue 421), I decided to comply by simply using a guard clause.

[codecov[bot]]

Codecov Report

Merging #435 (93424f5) into master (f11d685) will decrease coverage by 0.00%. The diff coverage is 87.50%.

@@            Coverage Diff             @@
##           master     #435      +/-   ##
==========================================
- Coverage   89.36%   89.35%   -0.01%     
==========================================
  Files          73       73              
  Lines        4381     4398      +17     
  Branches      701      707       +6     
==========================================
+ Hits         3915     3930      +15     
  Misses        338      338              
- Partials      128      130       +2     

Impacted Files	Coverage Δ
src/fastoad/io/variable_io.py	91.78% <80.95%> (-1.21%)	:arrow_down:
src/fastoad/openmdao/variables/variable_list.py	93.75% <100.00%> (+0.03%)	:arrow_up:

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