Cannot add extra arguments when inheriting `FGFDMExec`

[bcoconni]

As I was investigating @sinizu's request in discussion #516, I discovered that the commit 31ea0ff has never been cherry picked to the branch release/v1.1. So it is currently not possible to add arguments to a class constructor that inherits from jsbsim.FGFMExec.

For instance, the code fails for v1.1.11 but succeeds for the master branch

import jsbsim

class myFDMExec(jsbsim.FGFDMExec):
    def __init__(self, root_dir, pm_root, extra):
        self._extra = extra
        super().__init__()

[bcoconni]

The bug has been fixed by the commit e6e31f8.
Also added regression tests:

• master: commit ee15149
• release/v1.1: commit 1096d07