Avoiding self loops

[intigonzalez]

For now, we are triggering a warning if self loops exist in the protocol. This is because we are not handling them in the proper way. We can fix this problem in the following way:

• If the self loop is a default transition, then it is pointless
• If the self loop is a transition with predicate and time constraint, we can add a new urgent state.

For example: A - [b,t<4] -> A can be transformed to:

 A - [ b, t < 4] -> B*
 B* - [true] -> A