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.
Here we could invoke the Coach#players method that I commented, so we can use the names lists (and check their length).

I'm not sure whether we should pass the name to the Player instance or use them as keys in a hash ('players' attr being a hash rather than an array)