Bins data into nbin equal frequency bins, indexed from * 0 to nbin - 1, with 0 being the smallest bin, etc. * The values returned are the bin index for each element. * * Default return type is ubyte, because in the dstats.infotheory, * entropy() and related functions specialize on ubytes, and become * substandially faster. However, if you're using more than 256 bins, * you'll have to provide a different return type as a template parameter.