fix tests broken in 46893f6

It turns out that the change to log stuff into output/log.txt changed
the order of the header and the message of loading shared libs, which
broke 40 tests. This fix restores the original order and only prints the
header into the logfile as soon as that one is set up.
tests: numdiff should treat comma as a separator

This fixes parsing number of the form
 "Errors u_L1, p_L1, u_L2, p_L2: 1.274730e-02, 3.576354e-01"
correctly.
fix test outputs

A couple of tests changed slightly due to a different quadrature order
used in the KellyEstimator.
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