multiple baseURL's

multiple centres have expressed the desire to not only replace the baseURL on download, but add to it. the most obvious way to do that is to allow the baseURL
to be a JSON array, as well as a singular value. On the one hand, centres want DCPC''s to retain the responsibility, particularly in large datasets, for distribution, and in such cases, may elect not to re-write the baseURL at all. In other cases, to illustrate that there are multiple sources available, one would want to add the baseURL to the others in the list, so as not to monopolize distribution to downstream clients.

The issue names no files or tests. Start by locating where baseURL is read and rewritten during download, then inspect how JSON values are validated and serialized. Done means supporting both a singular baseURL and an array while preserving the requested behavior for replacing, retaining, or adding distribution sources.

Written by the indexing model from the issue text.