Add support for requires and excludes on the core features implementation in the fm_metamodel

[jagalindo]

Get core features from the cross-tree constraints (this takes a while)

requires_ctcs = feature_model.get_requires_constraints()

# new_core_features = core_features
# while new_core_features:
#     new_core_features = []
#     for ctc in requires_ctcs:
#         ctc_cnf = ctc.ast.to_cnf()
#         left_node = ctc_cnf.root.left
#         right_node = ctc_cnf.root.right
#         if left_node.is_op():
#             left = left_node.left.data 
#             right = right_node.data
#         elif right_node.is_op():
#             left = right_node.left.data
#             right = left_node.data
#         left_feature = feature_model.get_feature_by_name(left)
#         right_feature = feature_model.get_feature_by_name(right)
#         if left_feature in core_features and right_feature not in core_features:
#             new_core_features.append(right_feature)
#             core_features.append(right_feature)