Inconsistency between features objects and features strings in cross-tree constraints

[jmhorcas]

When storing cross-tree constraints in the feature model, the AST supports any type of nodes.
Currently, when reading the cross-tree constraints in the readers of the feature model the AST is built using the features' names which is OK, but then methods in the Feature Model class should return Features as objects, and actually there are some methods that returns the features' names such as the get_features() method inside the Constraint` class.

[jagalindo]

That's a change that might impact a bunch of places. This should be analyzed with calm