Add example of isolated galaxy and change hernquist potential configuration

In this merge request I have created two examples with an isolated galaxy, the first is an example that contains a static DM Hernquist potential and the second has DM particles according to a Hernquist potential.

Both examples use the same script to check whether angular momentum is conserved, and how much the scale lenght (h) and scale height (z0) change over the evolution of the galaxy.

The potential was changed that there is an option to run with an idealized galaxy disk, this means that the same parameters need to be given as in the parameter files of the code MakeNewDisk and GalIC. These are the bulge mass fraction and the baryonic disk mass fraction, besides either M200, V200 or R200.