Improve speed of source downloads

[bfhealy]

Complementary to #88, but separate because the download script runs differently. Iterating over the pages of existing sources takes a non-negligible amount of time before the download loop begins. Once it does, it is possible to download a few thousand sources per hour.

Feature Summary
To prepare for the need to download large source lists, we should streamline scope_download_classification.py to run as quickly as possible.

Implementation details
The page-by-page loop before downloads begin, along with the download loop that starts afterwards, are both good areas to focus for this enhancement.

[bfhealy]

With #90 implemented, if the user specifies a group from which to download rather than individual sources, it now takes a few minutes to download a few thousand sources from that group.