Remove generating_values from Grid class.

The property generating_values has been removed from the Grid class. Its use in the default constructor has been removed. Furthermore, the tests have been updated according to this change.

This change does not affect the class functionality but changes the way instances of the Grid class are created especially via the default constructor.

This MR should resolve Issue #148 (closed) and part of the refactoring of the Grid class (Issue #135).