allow seeding of vH and vE
demonstration of recurrence across string graph nodes

As of this commit, issuing the following command will demonstrate the ability
to seed an alignment with the H and E vectors from a previous alignment.

    ./ssw_example ATGC CTGA ATGCCTGA

This equates to a string graph of the form:

    ATGC -> CTGA

And we are aligning against a string that matches the entire string graph.

Several features which will be required for use in partial order / string
alignment are still missing.

In the case that a sequence node has multiple predecessors, we need to generate
the H vector as the maximum of inbound H vectors on a per-cell basis.  The
traceback can independently determine the direction taken by the optimal
alignment across the node boundaries.

The same will need to be done for the E vector.  On face value, this seems a
bit more complex and I am still nivestigating how this should be done.