rework linear solver tolerance

Change the Stokes linear solver tolerance to inlcude the current
pressure instead of only the rhs. In the past the formula for the Stokes
solver tolerance was:
   rhs.l2_norm() * parameters.linear_stokes_solver_tolerance

This causes problems if the magnitude of the rhs is large (for example
due to a large gravity vector) and if the current pressure solves the
problem fairly well (so the initial residual is much smaller than the
tolerance as defined above). This is basically caused by the static
pressure being a good solution. The new formula is:
  || B^T p_0 - G || * parameters.linear_stokes_solver_tolerance

This fixes the problem that one had to use a linear solver tolerance of
1e-15 to make convection_box.prm work.
update test output
update changelog
adjust linear solver tolerance for some cookbooks
update tolerance parameter description
Merge pull request #109 from tjhei/linear_solver_tolerance

Linear solver tolerance
fix new residual computation
fix test output again
fix petsc test
add a test for the direct parallel PETSc solver

which is using MUMPS right now.
split tests because PETSc and Trilinos now produce different output
Count number of tests correctly

also reindent code
Merge pull request #110 from tjhei/cmake_improvements

Count number of tests correctly
Make sure the comment settings only apply to prmfile listings --
because we output run time percentages in some of the listing boxes
which do not look nice in the manual if percent is a comment sign.
Remove 'frame=single' from two lstlisting environments since this is
already the default.
doxygen formatting, astyle, comment wrapping
release task: update version and changes.h
Merge pull request #111 from bangerth/fix-manual

Fix manual
update release tasks
Updated changes.h according to last pull requests.