Decoupled repl_ex_nst from nstcalcenergy

The replica exchange frequency is automatically changed by mdrun
to a multiple of nstcalcenergy, which is annoying. It turns out that
it doesn't need to be a multiple, so this changing has been removed.
Fixes #1494

Change-Id: I00833f92fd468924f61879aff8b7c85fe79d3c2e