I've put explicit called to `np.random.seed` in here already (prior to this PR).  There was some discussion between @astrofrog and I about this, and I came down thinking that including them explicitly (such that the reader sees them) was nice, because it makes "typing along" to the doctests reproducable.  But we could hide them if there's strong objection.  In any event, the seed should be being reset within each example already, which is why I was surprised to see the numbers changing.  I didn't know if that meant that some of the fitters were non-deterministic.