All elementary joints should inherit from this base model, i.e.,
joints that are directly defined by equations, provided they compute
either the rotation object of frame_b from the rotation object of frame_a
and from relative quantities (or vice versa), or there is a constraint
equation between the rotation objects of the two frames.
In other cases, a joint object should inherit from
Interfaces.PartialTwoFrames (e.g., joint Spherical, because there
is no constraint between the rotation objects of frame_a and frame_b
or joint Cylindrical because it is not an elementary joint).
This partial model provides two frame connectors, a "Connections.branch"
between frame_a and frame_b, access to the world
object and an assert to check that both frame connectors are connected.
CONNECTORS
