  gravity waves.</p>
<p>Initialize_implicit initializes constants for the implicit gravity wave computation.
  It is assumed that that all implicit steps are of length 2*delt and use
  the forward/backward parameter alph. initialize_implicit has to be re-called
  whenever either of these two parameters is changed. initialize_implicit should
  be called even if the explicit option is chosen for the gravity wave
  terms (the reference state temperature tref is subtracted from some
  terms anyway to reduce rou