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.
Make sure we search in the correct directory.
Change the order of two assertions.
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.
Robustify code.
Document being able to use $ASPECT_SOURCE_DIR.
Merge pull request #31 from gassmoeller/new_new_simple_compressible

Create a simple compressible material model.
Make the solidus test actually work.
Merge branch 'master' of https://github.com/geodynamics/aspect
Set a useful default for Reference compressibility.
Mention that we implement the stabilization
Merge pull request #36 from gassmoeller/new_new_simple_compressible

Set a useful default for Reference compressibility.
grammar fixes
various documentation fixes

This will make the latex stuff compile again
update parameters.tex
update .gitignore