Add generating function check to expand_dim() in Grid class.

Add generating function check to expand_dim() in Grid class

The expand_dim() method now works only with instances whose generating function to expand the dimension. If there is no generating function available and the dimension expansion is attemepted, an exception is raised. However, if the target dimension stays the same, then it does not matter if a generating function is available.

Additionally:

• a new property has_generating_function() is now available to quickly check if an instance has a generating function assigned to it.
• The test suite has been updated to reflect the new changes.

This MR should resolve Issue #151 (closed) and part of the refactoring of the Grid class (Issue #135).