Training: cache feature vectors for 2nd epoch onward

[StephenChan]

Cache feature vectors in the local filesystem if they were loaded from remote storage (S3 or URL).

I think this will do what it intends to, but I couldn't figure out a way to do it without committing OOP crimes somewhere (data_classes.ImageFeatures.load() in this case). I think the better way to do this would involve passing a Storage to TrainClassifierMsg instead of a DataLocation, and putting the temporary directory attribute onto that Storage. That probably involves a much larger refactor overall where the designs of Storage, DataLocation, and perhaps DataClass are reworked. I have ideas for that but I don't think I'm up for that for the remainder of the month.

Also want to make the caching optional (but on by default), so that it can be turned off in case filesystem space is a concern, as it may be for coralnet's largest sources (particularly since older sources have feature vectors about 8x in filesize).

Let me know if it'd be useful to merge this PR in the short term though.

[StephenChan]

Source	Images	Points/im	Epochs	Time for epoch 1	Time for each subsequent epoch
1097	1686	10	3	96s	3s
3354	1600	50	10	178s	8s
295	63263	10	10	66m41s	2m22s

The code may be ugly so far, but the results sure aren't... (remember, each subsequent epoch used to be as long as epoch 1)

[StephenChan]

Feature caching PR

[StephenChan]

Ready for review. Recent changes:

• Rebased on top of recently merged PRs
• Combined the two old commits (...since the second commit message had a typo which was bugging me)
• Added options for the caching behavior: OS-designated temp dir location, custom temp dir location, or no caching. No caching is useful if the caching could cause disk-space problems.
• Unit test improvement
• Updated README and CHANGELOG

Still want to refactor adjacent classes/methods for better OOP design, but not urgent and will leave that for another time.