Mission rework

[christophe-david]

This PR makes some change in mission module:

• making the interface with other models more consistent by:
  • making that FlightSegment._get_gamma_and_acceleration() uses a FlightPoint instance as input (partially addresses #404)
  • allowing to define the variable for reference surface area, set as an option of the mission module.
  • moving the definition of engine number to the propulsion model.
• reorganizing and completing unit tests in anticipation of incoming changes relative to #404.

[codecov[bot]]

Codecov Report

Merging #411 (4328e64) into master (24f6fbb) will increase coverage by 0.12%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #411      +/-   ##
==========================================
+ Coverage   91.90%   92.02%   +0.12%     
==========================================
  Files         203      203              
  Lines        8409     8413       +4     
  Branches      915      915              
==========================================
+ Hits         7728     7742      +14     
+ Misses        504      501       -3     
+ Partials      177      170       -7     

Impacted Files	Coverage Δ
src/fastoad/models/performances/mission/base.py	94.11% <100.00%> (+4.46%)	:arrow_up:
...ad/models/performances/mission/openmdao/mission.py	91.00% <100.00%> (-0.35%)	:arrow_down:
...s/performances/mission/openmdao/mission_wrapper.py	95.40% <100.00%> (+5.07%)	:arrow_up:
...s/performances/mission/segments/altitude_change.py	89.70% <100.00%> (ø)
...stoad/models/performances/mission/segments/base.py	93.29% <100.00%> (+2.79%)	:arrow_up:
...oad/models/performances/mission/segments/cruise.py	96.73% <100.00%> (ø)
...dels/performances/mission/segments/speed_change.py	88.88% <100.00%> (ø)
...stoad/models/performances/mission/segments/taxi.py	100.00% <100.00%> (ø)
...models/performances/mission/segments/transition.py	97.36% <100.00%> (ø)
...opulsion/fuel_propulsion/rubber_engine/openmdao.py	100.00% <100.00%> (ø)
... and 3 more

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