Work on link-to-node mapping

Implemented map_link_vector_to_nodes. It "works" but
there seems to be a problem with the underlying math.
Added fn to grid.base.py for vector projection

Function is find_true_vector_from_link_vector_pair(). Part of new code
for mapping link vector data onto nodes, broken into vector (x,y)
components. That way, we can output this and make vector plots.