Supporting closed kinematic loops

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Modelica automatically detects loops and replaces the frame_a.R ~ frame_b.R resulting from connect(frame_a, frame_b) with the residual equation. Initially, we can require the user to do this manually by calling connect_loop instead of connect. By storing some metadata in the frame connector if it belongs to a loop opening, like this

function connect_loop(F1, F2)
    F1.metadata[:loop_opening] = true
    connect(F1, F2)
end

we could detect the loop opening when we expand the connections and emit

residue(F1, F2) .~ 0

instead of frame_a.R ~ frame_b.R. @YingboMa do you have a suggestion on how to modify connection2set! to accomplish this? It's not apparent to me where the equality constraints between the potential variables are introduced.

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820073c models a fourbar mechanism with a closed loop

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• add cut joint example #55
  adds an example demonstrating a cut joint