gates are not opening and closing

[jbphet]

On the current master version, the sodium and potassium gates are not opening and closing when the action potential reaches the cross section. I suspect this is related to the recent conversion away from PropertySet, see phetsims/axon#102.

Assigning to @Denz1994, since he did the conversion.

[Denz1994]

The gates now open and close as intended. I am going to close this as this was reviewed with @jbphet.