Error in sphinx output + ""make doc"" failure on buildbot

See this question: https://answers.launchpad.net/yade/+question/255486

Some import issues. I also get some when running yadeSphinx.py locally on ubuntu 12.04.

Original bug report by Bruno Chareyre on 2014-10-08 : https://bugs.launchpad.net/yade/+bug/1378717