Make Alignment a StateNode

[rbouckaert]

At the moment, it derives from Map, which derives from CalculationNode, but not from StateNode, making it cumbersome to do any operation on the Alignment during MCMC.

Making Alignment a StateNode (or actually, making Map a StateNode) would solve this problem. Obviously, the TreeLikelihood still expects a static Alignment, but this functionality will be developed in a package.