Add a RPYMinJerk class

[pFernbach]

We need a new class inheriting from curve_rotation_t that implement independant minjerk formulation for roll pitch yaw.

It can already be done with a simple polynomial, but for our use case we need it to have a type that inherit from curve_rotation_t (it is necessary in order to use it as the rotational part of a SE3 curve).

[MaximilienNaveau]

This is the first step toward doing this: #112
I created a smooth SO3 trajectory that allows us to interpolate smoothly between 2 rotations.

Hence the above problem can be resolved easily using this new curve.

Remains to implement the inheritance for the SE3 curve compatibility.

[MaximilienNaveau]

I will probably update the above PR to handle the inheritance as well.

[MaximilienNaveau]

Actually we don't need it. So Once the PR is merged we close this issue.