add an option to add the NOSYS suffix to branches without systematics

This makes the n-tuple branches more consistent, and makes it very easy
to determine the list of branches for nominal.

It also means that if we add (or drop) systematics from a branch, the
names of the branches in nominal will not change.  That doesn't happen
all that often, but could still be helpful.

Leaving this disabled by default because it (obviously) changes the
n-tuple content.  Also we haven't yet decided whether this is even a
good idea, but it ought to be a good enough idea to allow users to at
least play with it.