Minor update

Added the file 'A_simple_example.m' as an example demonstrating how to use the HDG3d library - a script to solve the Poisson's equation on a cubic domain with Dirichlet B.C. Detailed comments are added to guide users going through the main procedures that are needed to finish a miniature error test of HDG3d.

Original release

Original release of the code by Francisco-Javier Sayas via his previous website under mathsci.udel.edu. Included documentation from the website.