Clean up advection-diffusion demo, disable viper interacion
Add disabling advection or diffusion via command line
Add non-split version of advection-diffusion demo
Correct docstrings in advection-diffusion demos.
Merge pull request #159 from OP2/adv-diff-nonsplit-demo

Advection-diffusion nonsplit demo

Clean up advection-diffusion demo and add non-split version.