Implement finite difference fallback
Seed with map estimator by default
Don't seed gpmsa test from the map estimator

This is because the lnValue function has an incomplete implementation when a
non-null gradVector argument is passed.  For now we'll just not seed at the
MAP estimator and we can fix this up later.
Update regression tests
Remove initial point member from Optimizer class

Make the pure virtual minimize() method take the initial point instead.  I
think this makes more sense from a design point of view.