Use a generator to get bytes of `pydicom.Dataset`s

[pchristos]

in store_instances of DICOMwebClient.

The store_instances method accepts a set of DICOM frames, dumps them to bytes, and, finally, encodes them into a multipart/related request.

The above process results in storing the original DICOM frames in memory 3 times:

1. as the original pydicom.Dataset instances passed into store_instances
2. as bytes encoded in store_instances
3. as the concatenation of the above bytes created in _encode_multipart_message

Using a generator in store_instances reduces memory footprint by 1/3, since the original pydicom.Datasets are dumped into bytes one by one while the multipart request body is being created by _encode_multipart_message.

[hackermd]

Thanks for your contribution @pchristos. Much appreciated. We will draft a new release and make a new version of the package available via pypi.