[IMP-dev] 2.1 release, last call

Assuming there are no hiccups with today's doc changes tonight, I'd propose we declare the current state of IMP to be the release (see the develop branch or the release/2.1 branch). Anyone who hasn't run their code on IMP from the last week or so (it hasn't changed significantly in that period) is highly encouraged to do so to make sure some recent fixup hasn't broken anything.