image_geometry documentation not generating

[ScottMonaghan]

As noted in PR #527, image_geometry documentation is not generating correctly.

I'd like to take a look and see if I can make some headway on this.

As of writing, the rolling image_geometry package on index.ros.org points to a API doc that only shows C++ API members and also lists the library version at 0.1, rather than current 4.1:

In the PR #527 @ijnek offered the following helpful advice, and this is where I will start.

...to auto-generate API docs onto docs.ros.org, there is a nice guide that came out recently. I haven't tried it out myself, but I believe it applies also for packages with C++ and python.

##Acceptance Criteria

• All public python members of the image_geometry API are documented along with correct deprecation info.

@ijnek, let me know if there's more you'd like me to look at here.