Add postprocessor viscous_dissipation_statistics
Added test and changed output for time_in_years
Included the pressure in the calculation for cases that are not completely incompressible
Removed lines that were commented out
Create a new statistics postprocessor that outputs basic statistics.
Added divergence of velocity
Remove /m from unit
Efficient calculation of divergence of velocity
Remove the basic statistics part from velocity statistics and update tests.

The tests can still reproduce their results, but they now need to call velocity statistics and basic statistics for this.
Merge pull request #55 from anne-glerum/postprocessors

Postprocessor that calculates the viscous dissipation
simplify implementation (also fixes test)
add documentation (mesh refinement plugins)
make compile with PETSc again

The direct solver was not working with PETSc. We pick MUMPS for now,
which seems to work okay but requires you set up PETSc with MUMPS for it
to work.
initialize variables to fix warnings
remove assert that will make direct solver fail in parallel
Merge pull request #65 from gassmoeller/make_basic_statistics

Make basic statistics
Add a postprocessor that computes statistics about the pressure. Add a simple Poisseuille flow test.
Merge branch 'master' of https://github.com/geodynamics/aspect
fix test outputs
Improved documentation and some code cleanup for steinberger material model.