Recommend consistent method for getting package version in python

[wjwwood]

In catkin packages the version is defined in the package.xml, and we should provide a mechanism for getting the current version in python (and perhaps C++), such that there is no duplication of the version number required.

This would be used, for example, by the catkin_* tools so that they can know the version of catkin being used at run time. See #331

This should work in install/devel space and probably source space (catkin_make is called before catkin has had cmake called). Additionally, it should pick the correct version in an overlay situation.

[dirk-thomas]

Since this is unlikely to be addressed I will close this ticket.

Currently the information from the manifest can either be accessed by parsing the manifest or in order to avoid that overhead the version number can be duplicated in the Python code.