Adds documentation to MoveClassLoader. Also removes the unused last_ field.
Adds SSMove class for sink-source moves.

Sink source mutations do not change the arrows in the quiver, except their
direction, so these preserve MMI-ness. However they cannot be checked using
standard MMI Moves, as they do no require a complete subquiver to be embedded.
Instead I have created a special move to handle this case. This will then be
added to the MoveClassLoader, so that when the class is computed, sink-source
mutations are also considered.
Adds the SSMove to the MoveClassLoader.

SSMoves consider the sink-source mutations. Up till now the MoveClassLoader did
not consider these at all, so would only show a subset of the move class. Now
they should give the whole class.
Adds an uneeded functor - None.

This is basically the same as not listing a vertex as connected, but I forgot
about that, so created this ConnReq functor that does not allow the connection vertex to have any arrows attached outside the submatrix.