Mission backward compatibility

[christophe-david]

This PR completes PR #430 by putting back compatibility with previous mission definitions, that had to assume an automatic addition of taxi-out & pseudo-takeoff phases at the beginning of the mission.

Sphinx doc preview: https://fast-oad.readthedocs.io/en/mission-backward-compatibility/documentation/mission_module/index.html (This was the occasion to spot a problem with environment creation in RTD. The change in docs/requirements.txt is a temporary fix)

[codecov[bot]]

Codecov Report

Merging #462 (37c5b89) into master (45e0a68) will increase coverage by 0.01%. The diff coverage is 93.54%.

@@            Coverage Diff             @@
##           master     #462      +/-   ##
==========================================
+ Coverage   82.84%   82.86%   +0.01%     
==========================================
  Files         118      118              
  Lines        5154     5171      +17     
  Branches      793      796       +3     
==========================================
+ Hits         4270     4285      +15     
- Misses        742      743       +1     
- Partials      142      143       +1     

Impacted Files	Coverage Δ
...sion_definition/mission_builder/mission_builder.py	87.12% <84.61%> (-0.24%)	:arrow_down:
...ion_definition/mission_builder/input_definition.py	92.22% <100.00%> (+0.55%)	:arrow_up:
...n_definition/mission_builder/structure_builders.py	94.00% <100.00%> (-0.04%)	:arrow_down:

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