Added NodeMatrix class.

This is an attempt at abstracting an underlying Node object data
representation behind a coordinate-based interface, which comprises
update(i, j, val), getFloat(i, j), getDouble(i, j), and getNumber(i, j)
methods. It's unclear how this should relate to Plate - I expected Plate
to be a concrete class which encapsulates a datastructure, but
NodeMatrix could probably be made to extend Plate without much effort.