[partial cherry-pick from b87a2b53b3]

Revised the unitests, added some doc and created a "check" compilation rule. All unitests passed ok.

Scores:
RNEA = 		4.5206 us
CRBA = 		5.29557 us
Cholesky = 	2.33944us
Jacobian = 	3.6358 us
COM+Jcom = 	3.95835 us

Adapted both the urdf parser and the romeo model to enable romeo inside pinocchio. CAUTION: Romeo is not standard any more and should be corrected, by enabling fixed joint in the urdf parser.