[minor][robot_wrapper] frameClassicAcceleration requires numpy

Fix bug related to new version of Eigen 3.3.5

Uniformization of git history between master and devel branches

And then change name of internal ADD_UNIT_TEST to ADD_PINOCCHIO_UNIT_TEST

Merge current devel branch into master for continuous git history.
Only commits on branch devel have been kepts, devel branch being the most advanced one.

Fix bugs in exp6

Documentation and unittest for forward dynamics with damped cholesky inverse

[algorithm/dynamics] add option to take damped inverse when doing cholesky factorization of JMinvJt

Improve efficiency of exp3 and exp6 operations

[core] Efficient implementation of exp6

This new expression comes from the fact that [w]x*[w]x = w*w.T -
\|x\|^2. For more information see "Differential of the Exponential Map":
http://ethaneade.org/exp_diff.pdf

Change differentiate into difference

Differentiate was not the right term as it is devoded to differential calculus.
This is related to #432.

Remove warnings in unit tests