Collections of refactorings for feature models

[jmhorcas]

I've incorporated a set of refactorings for feature models as M2M transformations from FM 2 FM.
This is the commit.

These refactorings allow to simplificate a feature model without changing its semantics. Current refactorings are:

• Mutex group
• Cardinality group
• Multiple group decomposition
• Or-group with mandatory feature
• Xor-group with mandatory feature
• Pseudo-complex constraint
• Strict-complex constraint
• Requires constraint (to be added)
• Excludes constraint (to be added)

Additionally, there are 3 news transformations (that are not refactorings):

• Deletion feature, which deletes a feature from a feature model.
• Commitment feature, which converts a feature in core feature.
• FM Feature Secure Names, which changes the names of all features (including features in constraints) for safe names using only ascii characters (or the charset provided).