New version of DEP 1

This is a more or less complete rewrite of DEP 1. I started with PEP 1, and
made as few changes as possible to modify it to fit our workflow. The
changes from PEP 1 are summarized at the bottom of the new DEP 1.

Thanks to Carl, Jannis, and Andrew for their input on this.

This PR also creates the new directory structure for DEPs, adds a DEP
template, and updates DEP 181 accordingly.
Should I just leave the aside out? It was a fairly important topic discussed here, that the shepherd is the one to ask rather than the author, so I'm trying to capture that justification here. But perhaps that's too inside-baseball-y and it could just be dropped?