Class hierarchy diagrams and basic class documentation

[agodemar]

We need diagrams of the up-to-date class hierarchy in JSBSim.
If someone did ever generate in the past a set of documentation files (Doxigen?), this is a good chance to revamp the procedure.
Class hierarchy graphics will go into the Programmer's Manual as well.

[bcoconni]

The class hierarchy diagram is automatically generated by JSBSim Doxygen docs. It is available at https://jsbsim-team.github.io/jsbsim/inherits.html

Note that each time a commit is pushed this diagram is updated by the jobs we run with Travis CI.

[agodemar]

Great, thanks!