flexible source dir

[rukyr97]

With this patch, users no longer need to move the full ns3-gym repo into the contrib folder of ns-3. Instead, they can use symbolic links. This wasn't possible without this patch because the code was dependent of the output of os.getcwd(). Herein, the default behavior remains the same. However, we can specify with the optional source_dir parameter, what is the working directory that we want to define as the source directory.

[pgawlowicz]

Thank you for explanation. Indeed, might be a nice feature for some users. Could you clean the commit history and provide only 1 commit with the feature? The two first commits are already in main repo, so just the code allowing to set the src dir.

[rukyr97]

It is done :)