Implement Commander: Professor X

At the moment, this is incorrectly implemented; Professor X can play up
to 3 cards regardless of the Villain/Adversary groups being used. It
should only work when there are at least 3 cards in that Deck with
"X-Treme Attack," which at this time means when any of Uncanny Avengers,
Uncanny X-Men, or X-Men First Class are being used.

In all 3+ player games, and 2 player games not using the Kree-Skrull
War, Professor X will lead X-Men First Class. The Solo Mode rules added
in the "Commander Abilities Linked to Specific Adversary Groups" section
of the Legendary: Villains rulebook do not give the Group being used
"X-Treme Attack".

The current cucumber tests assume the correct implementation; this
change makes some tests fail, and others pass, for a net change of 0
tests with different results.

[GH-15]