Discrepancy in types of document subjects

[juhoinkinen]

PR #501 was a quick fix for training SVC backend on fulltext corpus, but it did not address the underlying reason:

...DocumentDirectory defines uris for documents as a set, while in SVC it was assumed that uris are list or other subscriptable (which for DocumentFile is true)

Right now I am not sure if there is a reason for subjects to be a set from from DocumentDirectory but a list from DocumentFile, so I'm just reporting this as a possible bug.

[osma]

You are right, this is an inconsistency. It could be resolved either way. And it should be resolved, since it already caused a bug in a release!

It's quite common to assume that the order of subjects doesn't matter, and this is the case for all the current algorithms (SVC being a special case since it can handle only one subject per document). Also the vector representation for subjects is order-agnostic. So I think it would make sense to change DocumentFile to use a set instead of a list.