make the viewer more responsive when selecting/first showing a big array

[gdementen]

Currently it slows down to a crawl when working with big arrays.

The vmin/vmax computation could be the culprit. Being fast is more important than having perfect colors, so IF that is indeed the cause of the slowdown, we should use a sample, like for computing ndigits. But then the color code needs to be adapted to cope with having colorval > vmax OR, preferably updating vmin/vmax as we go, when loading more data.

[alixdamman]

Making viewer (editor) more responsive is not the purpose of implementing a "Data Buffer" as suggested in #33 ?

[gdementen]

The buffer for the visible data is indeed a part of the picture. But here, before we visualize anything we work on the whole array to compute the min and max values, which for largish arrays is too slow. And I am unsure computing the min/max only on the current buffer is a good enough solution. Having a global min/max that is at first computed on the buffer and "enlarged" as necessary when we get more data might be a good solution AND/OR do like we do for ndigits/scientific: compute it on a sample (pick a few hundred values from all over the array to minimize bias for "increasing" arrays)

[gdementen]

FWIW, here the slowdown I speak about is the time it takes between selecting the array in the list and the data being displayed. This issue is not about scrolling speed like #38.