Add robotbuilder library

which is a utility to generate the C++ source code of metapod models.

This lib is used to generate the benchmark models and is used by metapodfromurdf.

With the latter, one can also re-generate the simple_humanoid and simple_arm sample models.

This branch also fix a bug in spatial. Apparently, my comment on
https://github.com/laas/metapod/commit/30d3878259dd4b60a1b88d9535fbf7b1d0bc2e4c
went unnoticed.

We probably need better test coverage for spatial too.