Break apart update_constraints

Break apart update_constraints

There are four distinct kinds of work being done, and never was any
call to update_constraints doing all of them, so it's better to have a
group of functions, each of which do one thing, and the relevant ones
called. This also makes it simpler to express by returning fast that
when we don't have constraints, we do nothing.

Made the logic for whether this is a log or energy step match that of
the main MD loop. The old implementation may not have prepared for the
last step correctly when it was triggered by something other than the
nsteps inputrec value.

Removed a commment mentioning iteration, which is a feature
that was removed a while ago.

Removed some ancient debug dump output.

Refs #2423, #1793

Change-Id: I21c10826721ddc9a79a33b1dc75971a20d0855d9