Multiple parallel environments now possible with latest JSBSim version

[opt12]

Just for anybody who is interested: The restriction, that it is not possible to have multiple instances of the environment is now gone with the latest rolling release of JSBSim.

Thanks to @bcoconni the underlying bug in JSBSim was fixed last Sunday with JSBSim-Team/jsbsim@2661d7c

As this resolves JSBSim-Team/jsbsim#201 it is now possible to instantiate multiple environments which is very common in several ML algos.

[Gor-Ren]

Thanks for the heads up! I've updated the README.