Feature/optimizedmatrix - One Column + graph propagation test

right-propagation of a row not present - would that need to be in a special case class and override propagate?

For the test case - https://github.com/tomtau/scalding/issues/18
Shall I "emulate" it in Matrix2OptimizationTest with an empty pipe with a set k (or random?) as optimize((G * G * G ...).propagate(V)) should be Product(Product... ?