Refactor ProgressRecorder for non-optional Celery usage and add doc strings

Close #377 (closed)

added part::backend type::refactoring labels

@debdolph I added your branch for !491 (merged) as target, so you can merge this into yours after @nik did a review.

added 7 commits

• 0bdf5664 - Remove questionnable workaround for timing issue
• c81581cc - Simplify and rename task recording decorator
• a8400ce4 - Update documenttion on ProgressRecorder
• 0d35f974 - Factor recorded_task out of class for brevity
• f2e29d79 - Rework progress API to be more coherent with upstream and DRY
• aa176305 - Add iterator interface to record progress from an iterable
• 924b33a7 - Remove unused import

Compare with previous version

resolved all threads

mentioned in commit d7b52b1b

merged