Implements Hesthaven textbook expressions into the code base.

It can be useful to have an implementation of the Matlab codes in Hesthaven's book.  A contribution could be to add this following a similar code style of what has been already done. This can be a good exercise to practice DG, python and github procedures.

For reference, this repo contains those codes in Matlab: https://github.com/tcew/nodal-dg