Set sys.path in wrapping conf.py so custom conf.py does not need to

[rkent]

This PR came about because I was trying to figure out why launch did not generate any useful python documentation.

Setting the python directory is tricky for the user, combining both a wrapping directory as well as conf.py directory depth. No real reason they need to do that though, we can do it for them (but they could still override it in their conf.py if they want).

This does not fully solve the problem with launch, as they have another problem in their custom conf.py which I will address there in a PR. But this might help #120

[rkent]

As I am looking at packages that are struggling with rosdoc2, I cam across another reason why this PR is important. The recommended way at the moment to set the python path in conf.py is with a complicated statement like:

sys.path.insert(0, os.path.abspath(os.path.join('..', '..', '..', '..', 'cv_bridge', 'python')))

The problem though is that this depends on where rosdoc2 is run from. If you run it from the repo, or you run it from the package directory itself, changes what the above should be. That is clearly not something that is acceptable, so this needs to be set in the wrapping conf.py (as in this PR), not the user conf.py

[rkent]

After more thinking about the sys.path issue in conf.py, I realized that there really is no reliable way to do that in conf.py. So I changed the code comment in a followup commit that discourages setting it there.

[tfoote]

Thanks this looks like a good way to avoid requiring user customization.