Fixed minor bug in the controller
Fixed minor bug in the controller

There was a typo in the str method of the controller. It was causing
some problems for printing the controller especially if there was any
mixture phase in the simulation.

TomTranter/OpenPNM,amdouglas/OpenPNM,stadelmanma/OpenPNM,amdouglas/OpenPNM,PMEAL/OpenPNM