Fix union when we've already got a non-single-run data collection

[takluyver]

@daviddoji relayed an error with code like this:

run = open_run(prop, run_nums[0])
for run_num in run_nums[1:]:
    run = run.union(open_run(proposal, run_num))

After the first .union(), the DataCollection object is not a single run. Calling .union() the second time failed because it tried to get self.run_metadata(). This should fix it by always checking .is_single_run before calling .run_metadata().

cc @kakhahmed

[kakhahmed]

Thanks for the fix, didn't think of that situation.

LGTM

[takluyver]

Thanks!