ft xml fk

[maxspahn]

• ft[xml]: Initial commit for xml fk.
• ft[xml-fk]: Adds initial implementation of xml fk, see XML Fk #36.

Mujoco XML's are getting increasingly more used for robotics. This PR brings
the first functionality of urdf fk to xml files.

[maxspahn]

This is only an initial implementation that still lacks some features.
Let's see how it works in practice before we add more features and settle on an
implementation.