TST: Show that histogramdd's normed argument is histogram's density

[eric-wieser]

Relevant to gh-4371

[charris]

Thanks Eric. To bad we needed to replace normed with density for histogram

[eric-wieser]

As a follow-up, I was planning to rename the argument to histogramdd from normed to density (deprecating the old name). (edit: done in gh-11531)

On the other hand, this just creates churn if we're also going to consider #9921 and come up with a different argument naming scheme (which I'm -0.5 on because I think people will choose a pmf when it's innapropriate if we make it too easy)