set ASPECT_USE_PETSC for test plugins if necessary

this fixes the problems that plugins would think they have to compile
with Trilinos even though ASPECT is compiled with PETSc.
have separate plugins pick up ASPECT_USE_PETSC

Same problem as with the test plugins. Now the define ASPECT_USE_PETSC
will be set correctly if ASPECT was compiled with PETSC support.
Merge pull request #185 from tjhei/fix_petsc_plugins

Fix plugins when using PETSc
Use a random number generator that we know is predictable.

The previous version used drand48 which is not thread-safe and whose
implementation can, at least in principle, be different on different
systems. The new version is more predictable, using a member variable
and a known random number generator from the BOOST library.
fix second place that uses old random number generator
update test output
Merge branch 'bangerth_make-particle-generation-more-predictable'

(this supersedes pull request #188)

The previous version used drand48 which is not thread-safe and whose
implementation can, at least in principle, be different on different
systems. The new version is more predictable, using a member variable
and a known random number generator from the BOOST library.

Also fix test output related to these changes.
fix test output
Merge pull request #187 from tjhei/fix_test_output

fix test output
Bugfix and additional rotation angle output.
Merge pull request #186 from gassmoeller/gplates_output_fix

Bugfix and additional rotation angle output.
Fix a bug and add a test.
Merge pull request #189 from gassmoeller/fix_adiabatic

Fix a bug and add a test. We had a bug in the initial condition 'adiabatic' that only got triggered if one used a model with the bottom boundary layer parameter set to some value and without adiabatic heating. The temperature difference between model and boundary temperature was calculated wrong in this case. This pull request should fix this bug and add a test for this case.
Add all of the files that describe the Davies et al. benchmarks.
Cleanup some of the merge problems with this patch by undoing some changes.

Undo accidental changes originating in a botched merge on Joey's branch.

Remove the free surface description. This piece needs to move to a different
branch and pull request.
Add forgotten files.
Rename the directory for a plugin.
Edit the plugin files to match our usual standards:

Reindent and minor edits. Properly declare that the viscosity depends on the temperature.

Merge the .h and .cc files for this plugin.
Go through the new section and edit and revise it. Make the input files in the doc/ directory only those pieces that really matter.
Finish editing the manual:

Slight edits in explaining the process of loading benchmark shared libs.

Minor edits.

Reference a particular section.

Document some of the results.