Document the order of the elements in the tangent space, in a prominent place

[joansola]

An important detail in the usage of manif is the meaning of the Jacobians. In particular, the order of the different elements in the tangent vector is crucial.

For example, in SE(3) we may have [theta,p] or [p,theta]. Mathematically, this is irrelevant. But code-wise, the order matters.

In SGal(3) there are 4 elements, so the number of options is huge, and only one is valid, It happens that the selected order (rho, nu, theta, t) does not match the order in the constructor of SGal3 (p, q, v, t), leading to possible confusion. The only good solution is a good documentation.

The arbitrary choice of these orderings should be clearly documented in a prominent place (e.g. in the GroupTangent classes), and appear in the documentation pages generated by doxygen.

I also noticed a few Error404 when browsing the doxygen documentation, especially when looking for specific files in the SGal3 class impl folders.