Ask whether to open a new dataset replacing the current, in non-additive loads

[dbenn]

Add dialog: "Do you really want to load a new obs dataset over the current one?"

This came from a broader context: #194

[mpyat2]

That's a good point. Also, it would be good to reset VStar after the "Yes" answer to this question, such as to clear the plot area, etc. Currently, if an error occurs while loading data, the old data remains, but some of the buttons become inactive.