More control over formation changes.

Allow to use :average1 .. :average6 and :fast1 .. :fast3 to allow more control over what player moves to a newer position.
The old way a single change in movement would cause a lot of movement and often leave the team vulnerable.

NOTE: This change is backward compatible. i.e.: the old way of specifying formations will still work.
Looking at this again, and after discussing with @jmbejar, it seems this logic should me moved to the coach class.