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.
Add an entry.
Merge pull request #190 from bangerth/WDurkin-davies-benchmarks-2

This patch has all of the files that are required to reproduce the first few of the Davies et al. benchmarks, plus a section in the manual that describes these benchmarks.
optional perl script to postprocess a screen-output file
use .sh to make tests/burstedde work with clang and gcc
also track dependencies for the new .sh test scripts
Merge pull request #191 from tjhei/test_py_postprocess

Optionally run <testname>.sh script to allow replacing the screen-output for more robust test comparisons. Use this method to fix tests/burstedde that gave different results between gcc and clang.
fix test output
Merge pull request #192 from tjhei/fix_test_output

fix test output
Work around a problem with deallog when loading shared libs.
Add the files necessary to reproduce the variable viscosity, dynamic topography computations.
Add Joey's patches to the manual (which came from a separate patch).
Move declaration into the .cc file.
Clarify documentation and minor improvements of a line of code.