Completed unit testing the Cell class. An interesting issue came up with the example sphere that was designed. In the BooleanCellFunctor member functions, some of the precondition tests assumed that at least one set operation must be present. The shere is a clear example of a correctly specified cell that does not contain any set operations. The preconditions were eliminated and the member functions now check for the special case of zero set operations being present. A Unit test should be added to the BooleanCellFunctor unit tests to test the sphere case.