Compute radial and tangential components of velocity / acceleration

[sfmig]

Is your feature request related to a problem? Please describe. There are a few reference frames in kinematics/dynamics that are often useful, for example to relate kinematics to forces, or to express a kinematic quantity as components that are more interpretable.

For example (they may have better names):

• the normal-tangential basis(also called intrinsic coordinates, also Frenet-Serret frame) -- nice diagram for the velocity case here
• the increasing rho-increasing theta basis (confusingly sometimes called polar? nice explanation here),

Describe the solution you'd like One option could be to treat these as change of basis operations. We could have a function that computes at each timestep the change of basis matrix from "world" coordinates to "intrinsic" coordinates. Then this matrix could be applied to the velocity or acceleration vector to express it in intrinsic coordinates.

[niksirbi]

I like it, especially the normal-tangential basis.