Change core to cyclic graph using mixins about brim HOT 2 CLOSED

This is all quite fun. The thing when you really reduce this, then you'll just get to the level of individual joints and bodies, which is just the high level API (which is partially still getting implemented) in sympy.physics.mechanics. If one wants that then (s)he should just use that. At the same time we are using it internally, so it is of course compatible with it.

Overall it is nice to support mixins, to make sure a model gets compatible with something, where one can even use that mixin to define the kinematics between the two. This is especially nice for the interactions between the bicycle and the rider. One can however also group a set of model connections as one may know that they will be used together. This is what WhippleBicycle does.

One can pose the following rule on splitting:

• Keep it together if one will just almost always use it together.
• Split it, if you'd like to be able to changeout parts easily. A good example is the different tyre models and toroidal and knife edge wheel.

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A possible concept to fit this approach would have been having a group of models, like wheel models, etc. And a group of interactions, which each describe how two models can be connected. Next a user can choose what models should be combined and then it just searches what valid interactions exist.

While this approach would have some fun effects around certain flexibility, it would become really messy to try and track down what exactly happened, as the dynamics code will be partially defined on a lot of places.

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