Improve MGM2Mind by adding a Scala representation of Offer, Move, UnilateralMove and BilateralMove

This merge request includes an improvement for MGM2Mind based on:

• an abstract class representing a move
• a class representing an unilateral move
• a class representing a coordinated bilateral move
• a class representing an offer
• some functions for comparing floating-point numbers in Scala
• the method relevantConstraints(constraints: Set[Constraint]) : Set[Constraint] of the class Variable