add ability to cancel long running data requests
From Sheila:
Another dataviewer feature that I miss is that when I made misguided requests from dataviewer, (asking for 1 year of 16kHz data), I could hit the stop button, and it would stop but not crash. I often crash ndscope or make my session unusable with a stray zoom. I think that I've talked about this with Jamie and there was some reason why this isn't possible using nds2, but I might be misremembering.