add Burstedde benchmark

This is a combination of 10 commits made by @bangerth in #155:
Using files by Iris van Zelst (with only minor edits), add whatever is necessary to reproduce the Burstedde benchmark.
Massage the previous patch into a form that fits in the manual. Also externalize the input file to make sure it gets automatically annotated.
Adapt the benchmark files. Minor edits elsewhere.
Make Iris van Zelst's benchmark into a testcase.
Add an entry.
Re-indent.
Fix descriptions of plugins.
Adjust output to that on the tester.
Revisit and clean up these plugins.