Grid model

Implements GridModel functionality. Includes testing for initialization of GridModel, size queries, querying of cells (isStart, isGoal, openings) via GridModel. Includes testing for connections between two cells. Added testing initialization methods to GridGenerator and GridModel to initialize with a string and grid, respectively.
When we refactor for style later, I think this method could be clarified a bit. Namely, I'd be able to read it more easily if there were more variables declared for readability. (For instance, this line has lots of brackets.)