feat asyncio dicom export

[fordmcdonald]

Speed up XNAT DICOM fetch in dicom_export with asyncio/aiohttp

Summary

This PR parallelizes DICOM downloads from XNAT during the dicom_export phase of xnat2bids by using a single-threaded asyncio event loop with aiohttp. Files are streamed to disk concurrently and then BIDS-ified, reducing wall-clock time on exporting session DICOM scans.

I'm recycling the authentication credentials and session cookie from the prior connection set up via the requests module.

Another point of confusion might be the semaphore created in the run_remaining function. This is passed to the Task method download_and_bidsify to limit the number of concurrent requests to 30 per call to xnat2bids with the reason being to soften the load on XNAT. In previous testing, I found XNAT's throughput bottlenecked around 64 concurrent requests.

[hollandjg]

The code looks great! I've a couple of questions, but it's neat and clean and understandable.

My comments are mostly questions about the dependencies, and whether you need all of the ones added, given they aren't imported by bids_utils.py.

Thanks for this, Ford!

[hollandjg]

This looks like a file which wasn't used anywhere in the repo. What was it doing here?

[hollandjg]

Great! Sorry for the slow re-review!