Merge pull request #294 from ufo-kit/add-directory-walker-test-and-fix-293

Add directory walker test and fix 293
Make dsetname public
Use reasonable default dsetname
Update experiments for new walker behavior
Fix walker test
Allow more data sets in one DirectoryWalker level
Test multiple data sets in DirectoryWalker
Unify error messages about nonempty data sets

DirectoryWalker now extracts the dsetname from the formatting string and outputs
that instead of just the current directory.
Don't waste memory in walker tests
Merge pull request #295 from ufo-kit/fix-walkers

Fix walkers
Add control loops
Add dummy loop
Add control tests
Add control to docs