Removing simpleCell

[ChasingNeutrons]

We now have unionCell which is essentially a superset of simpleCell, i.e., it reduces to simpleCell when there are only intersections. It is not necessary to duplicate the code, therefore we should remove simpleCell. As well as deleting the code, the main task is to change old input files which feature cells.

One thing we might want to do is to have unionCell behave like simpleCell in being able to read surfaces as a list, rather than as the new tokenArray object (again provided it contains only intersections). This would make changing all of the old inputs quite straightforward.