Error norm plotting

[stbachinger]

This PR adds a new post processing script to the EDGE repository. It takes a directory of error values of the structure:

It creates convergence plots for the different characteristic lengths. It expects data files to be of the following structure: /errors/${timestepping}cl${cl}pa${parallel}.xml, where timestepping can either be gts or lts, cl a value in between 1-10, 15, 20, 25, and parallel to be 1 or 13.