Prevent ensemble property of moves object from being edited

[rsdefever]

Prevents the ensemble attribute of the moves object from being edited after the moves object is created. The moves object contains default selections that are made based upon the ensemble that is selected when the object is initially created. Instead of allowing the user to edit the ensemble after the moves object has been created, this PR instead requires that the user create a new moves object if they wish to change ensembles.

Closes #50 , related to #3

[rmatsum836]

This looks good to me! I think preventing ensemble edits makes it much easier to verify that everything is correct before running a simulation.

[rsdefever]

Thanks for looking it over! Pretty simple change but I think the behavior makes sense.