Create a simple compressible material model.

Add test for simple compressible material model.

Updated documentation for simple compressible material model.

Added functions to simulator access that return the surface pressure input parameter.

This is a very simple compressible material model (in fact even simpler than the simple model, because the viscosity is constant and compositional fields have no effect on everything).  I need it for testing and benchmarking purposes but I think it is good for illustrating how to use compressibility as well.