Move write_images to concert.storage

because of a future circular dependency between
coroutines.sinks and storage modules
Don't append file extensions by writers
Add file system walker

and image-based implementations. Fixes #259.
Update experiments to use walkers
Fix pep8 in concert.experiments.base
Update experiment tests
Merge pull request #260 from ufo-kit/generalize-acquisitions

Generalize acquisitions