Added vonmises mat - viz works without EPD

I added a python von Mises plasticity model that allows
combined isotropic/kinematic hardening. The model is not
very pretty (which highlights some possible work to the
interface). I also hastily made one simple benchmark for it.

I also put in a 'try:...except:" block into the viz system
so that I could use it without installing EPD (so only using
packages installed from the Debian/Ubuntu package manager).