PUB-416. increases Summary2 quantile estimation accuracy. There was/still is a heuristic for picking bin size for Summary2 histogram, by looking at min/max vals in a column. In some cases it only created 3 or 4 bins. This was bad for quantile estimation, since that uses the histograms. Since for some data the heuristic ended with 20 bins, I decided to force the bin count up to 20 if the initial heuristic didn't. This is done by adjusting the proposed bin size, and iterating till the resulting number of bins is >=20. (the heuristic is odd enough that I couldn't always get it in one adjust). And the bin cnt is deduced from bin size, and I didn't want to mess with that, to avoid undersizing the histogram range

example of adusting binszTry, given _max2 and start, (and resulting nbin)

attempt 0 nbin: 3 binszTry: 50.0 _max2: 100.0 _start: 0.0
attempt 1 nbin: 14 binszTry: 7.5 _max2: 100.0 _start: 0.0
attempt 2 nbin: 20 binszTry: 5.25 _max2: 100.0 _start: 0.0

another col:
attempt 0 nbin: 11 binszTry: 500.0 _max2: 99.89928165450691 _start: -5000.0
attempt 1 nbin: 19 binszTry: 275.0 _max2: 99.89928165450691 _start: -5000.0
attempt 2 nbin: 20 binszTry: 261.25 _max2: 99.89928165450691 _start: -5000.0

really should go to 100 bins, but didn't want to overburden the browser summary page (which shows the histograms)