Refactored GridBuilder_Specs to use helper methods

Learned how to do helper methods in NSpec---helper methods should not
use underscores
Make GridBuilder build grids :)
Extracted method PositionInGrid.ToScalarForGridOfSize(...)
Extracted method PositionInGrid.FromScalarForGridOfSize(...)
Extracted method PositionInGrid.IsOutOfBoundsForGridOfSize(...)