Fast-oad

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- Added compatibility with OpenMDAO 3.2.
- Added the mission performance module (currently computes a fixed standard mission).
- Propulsion models are now declared in a specific way so that another
module can do a direct call to the needed propulsion model.

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- Prevents installation of OpenMDAO 3.2 and above for incompatibility reasons.
- In Breguet module, output values for climb and descent distances were 1000 times
too large (computation was correct, though).

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Some changes in mass and performances components:
- The Breguet performance model can now be adjusted through input variables
in the "settings" section.
- The mass-performance loop is now done through the "fastoad.loop.mtow"
component.

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- Adapted the FAST-OAD code to handle OpenMDAO version 3.1.1.

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- In Jupyter notebooks, VariableViewer now has a column for input/output type.
- Changed base OAD process so that propulsion model can now be directly called
by the performance module instead of being a separate OpenMDAO component (which
is still possible, though). It prepares the import of FAST legacy
mission-based performance model.

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- Changed dependency requirement to have OpenMDAO version at most 3.1.0
(FAST-OAD is not yet compatible with 3.1.1)