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.
Changed the compressible formulation to be consistent and updated test results.
Add another test that ensures the simple compressible material model gives the same result as the existing compressibility_iterated_stokes test.
Document variables and remove a now unnecessary variable.
Code refactoring.
Add documentation to a test.
Merge pull request #31 from gassmoeller/new_new_simple_compressible

Create a simple compressible material model.