Use `Vector{Motion}` instead of `MotionList`

[pvillacorta]

At the moment, the phantom motion can be defined as follows:
obj.motion = MotionList(Translate(...), Rotate(...), ...)
We would like it to be defined like this:
obj.motion = [Translate(...), Rotate(...)],
so there would be no need to define the MotionList and NoMotion structs.