Opening the run in the main thread blocks the GUI

[takluyver]

If opening a run takes a long time (as it not infrequently does), the GUI locks up until it's done. Besides efforts to make opening a run quicker, we should possibly also look at loading data in a separate thread, so that the GUI can work and show a spinner or something until it's done.

h5py currently doesn't release the GIL, but even without that, Python should give the GUI thread chances to run. h5py 3.0 will have the ability to release the GIL on some operations.