Add slerp and override getattr method

[Giulero]

This PR introduces the slerp method.
See the example slerp_so3 to check how it works.

Also, I introduced a small modification that would allow passing directly Quaternion objects to casadi methods without accessing the quaternion coefficients with .coeffs():

    def __getattr__(self, attr):
        return getattr(self.xyzw, attr)

This modification should give to the Quaternion class methods from the underlying type that is used to build a Quaternion instance.

For example, before:

opti.subject_to(quat[k + 1] == (rotation_SO3[k+1]).as_quat().coeffs())
# or
opti.subject_to(quat[k + 1] == (quaternion_obj[k+1]).coeffs())

Now:

opti.subject_to(quat[k + 1] == (rotation_SO3[k+1]).as_quat())
# or
opti.subject_to(quat[k + 1] == (quaternion_obj[k]))

This modification shouldn't break the previous code since the .coeffs() method is still accessible.

A lot of doc is still missing! I'll add it in a different PR.

[Giulero]

Thanks @FabioBergonti !

[diegoferigo]

@Giulero sorry for the late feedback, it looks good to me. Is there any reason why you removed the quaternion product method? I don't think I'm using it, but it might be useful to others.

[Giulero]

Is there any reason why you removed the quaternion product method? I don't think I'm using it, but it might be useful to others.

Actually, it was an old duplicate in SO3 class.
The quaternion product is in the Quaternion class :)

[diegoferigo]

Is there any reason why you removed the quaternion product method? I don't think I'm using it, but it might be useful to others.

Actually, it was an old duplicate in SO3 class. The quaternion product is in the Quaternion class :)

Ow ok it makes sense! Thanks for the detail 😉