Property setter for Gate model

[gingo00]

There is a property for gate model in cellengine.payloads.gate, but there is no setter for it. Modifying the munch object does not update the model within the gate object:

#Does not update mygate.model, because the model property returns a new Munch object
mygate.model.rectangle.x1 = mygate.model.rectangle.x1 + 1

#Error, no setter for the property
model = mygate.model
model.rectangle.x1 = model.rectangle.x1 + 1
mygate.model = model

Being able to update the model like that would be helpful to move the gate and then call gate.update() to persist that modification to CellEngine