Add an option --outputpath to the JSBSim scripts

[bcoconni]

This PR adds an option --outputpath to the binary executable JSBSim and the Python script JSBSim.py. This allows to specify from the command line the output path where all the output files will be written.

I took this opportunity to:

• change the order the arguments are displayed in and group them by themes (input, output, miscellaneous)
• Fix the Python module name (was _jsbsim but should be jsbsim). For some reason, no one complained about that, most likely because JSBSim.py is not known or used.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 25.10%. Comparing base (d89e357) to head (bfdcd3d).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1478   +/-   ##
=======================================
  Coverage   25.10%   25.10%           
=======================================
  Files         171      171           
  Lines       18843    18843           
=======================================
  Hits         4731     4731           
  Misses      14112    14112           

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[seanmcleod70]

Looks good.

[bcoconni]

Thanks for the review.
PR merged.