Refactor accessing pinocchio robot via assembly instead of saving direct reference

The default access to the Pinocchio robot should be via the module assembly. This would clarify that Timor is the main interface and Pinocchio robots are only one of many possibilities.

It will also make the serialization of solutions easier and keep the assembly of the robot outside the Pinocchio robot wraper (see !21 (closed)).

In general constructs like:

r = assembly.to_pin_robot()
r.fk()

should become assembly.pin_robot.fk().