Add support to open run data from any number of data locations

[philsmt]

Somewhat coincidental to our internal discussion on differentiating data from raw and proc, I already planned to extend the data argument of open_run to allow specifying multiple and arbitrary locations relative to the proposal root. The main use case is the ONC offline corrections, which by default output to <proposal>/scratch/cal. With this PR, you could open this like:

run = open_run(proposal=123, run=456, data=['raw', 'scratch/cal'])

The previous case data='all' is implemented as an alias for ['raw', 'proc'], and it preserves the overwriting rules we established for it.

[takluyver]

Thanks! This looks good, but I might be inclined to hold it until we can look data up by origin in the case of clashing names as well (something like run.get('MID_DET_AGIPD1M-1/DET/0CH0:xtdf', 'image.data', data='proc')). That's going to require a more involved change, and if we're not careful we might accidentally make it harder to deal with that.

[philsmt]

Do you think we're still going for some layer implementation or we can continue here?

[takluyver]

I've rebased this onto current master. Now that loading aliases no longer prints a message (#478), I've also simplified the logic a little bit to just load the aliases again for each origin.

@philsmt would you have time to write a test for this?

[philsmt]

Thanks, I did! The diff looks weird, but essentially I moved the bits testing data='all' from test_open_run into test_open_run_multiple parametrizing all and ['raw', 'proc'].

[takluyver]

Thanks, LGTM

[philsmt]

Thanks for review, probably my MR with the longest review time. Could've waited the two weeks to make it 2 years 😁