Added a feature for concurrently running several computations

[christophe-david]

This PR adds the CalcRunner class:

• it allows to "make local" FAST-OAD computations, i.e. put in one folder all input and output files, making it possible to run again the computation with only this folder (configuration file is modified accordingly)
• it allows to run a batch of these "localized" computations concurrently, using either multiprocessing or mpi4py.

The follow-up of this PR will be:

• Making this feature available from CLI
• Refining the continuation of batch, based on calc results instead of simply the existence of the calc folder (based on calc number)

In a second time, adding a way of building the computation inputs (i.e a DoE) will be useful.

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 85.00000% with 18 lines in your changes missing coverage. Please review.

Project coverage is 91.69%. Comparing base (fe49814) to head (531d5c9). Report is 3 commits behind head on master.

Files with missing lines	Patch %	Lines
src/fastoad/cmd/calc_runner.py	76.81%	9 Missing and 7 partials :warning:
src/fastoad/io/configuration/configuration.py	97.56%	0 Missing and 1 partial :warning:
src/fastoad/openmdao/problem.py	88.88%	1 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #554 +/- ## ========================================== - Coverage 91.84% 91.69% -0.15% ========================================== Files 97 98 +1 Lines 5957 6070 +113 Branches 931 955 +24 ========================================== + Hits 5471 5566 +95 - Misses 328 338 +10 - Partials 158 166 +8 ```

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