Remove moment_partial from qp.Ensemble

[drewoldag]

In the qp.Ensemble class there is a method for calculating moments called moment_partial it doens't seem like it really belongs there since there aren't any other metric related methods in the Ensemble class. It also makes for a strange dependency graph. Additionally there are other ways to calculate the moments for an ensemble of PDFs. And finally, even in the demo notebooks, the moment_partial method is not shown.

I would advocate for removing it, however, I don't know if there are historical reasons for keeping it around.