Data Explorer: Add support for Freedman Diaconis histograms

[dfalbel]

This PR adds support for the Freedman Diaconis algorithm for computing the histograms. It also adds a simple rule to make histograms for integer variables better: if num bins proposed by the binning method is larger than the width of variable range (data.max() - data.min()), then we use the integer range as the number of bins.