Clear sweep table when choosing a new nwb file

[sgratiy]

Currently when selecting a different nwb file from menu action, the sweep table will show sweep data from the previous nwb file until the auto_qc features are calculated for new dataset. Also, the file open dialog does not close until the auto_qc features are finished calculating.

Desired behavior:
Upon selection of a new nwb file: file open dialog needs to close, sweep table view needs to be flushed.