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.
Rename the directory for a plugin.
Reindent and minor edits. Properly declare that the viscosity depends on the temperature.
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.