stat/distuv: incorrect length of suffStat parameter passed Normal.Fit

[dongweigogo]

For normal distribution, the suffstat obviously has a length of 2, with mu and sigma. But in the distuv/norm.go source code, the length of suffStat in the Fit method is set to 1. That causes a panic.

func (n *Normal) Fit(samples, weights []float64) {

    suffStat := make([]float64, 1)                  //HERE

    nSamples := n.SuffStat(samples, weights, suffStat)

    n.ConjugateUpdate(suffStat, nSamples, []float64{0, 0})

}

I think that should be set to 2.

[kortschak]

Presumably it should be written

func (n *Normal) Fit(samples, weights []float64) {
        suffStat := make([]float64, n.NumSuffStat())
        nSamples := n.SuffStat(samples, weights, suffStat)
        n.ConjugateUpdate(suffStat, nSamples, make([]float64, n.NumSuffStat()))
}

I would think NumSuffstat deserves some documentation too.

Also fix Exponential with the same code.

[kortschak]

@dongweigogo Please send a minimal reproducer for testing.