fix: outgoing vertical coordinate should be 'height' in meters using zGridCenter

fixes #524

Following the call tree, using oncenters always

convert_vertical_obs
  -> convert_vert_distrib
     case (VERTISHEIGHT)
     -> find_vert_indices
        -> find_vert_level(oncenters = true)
           case (VERTEIGHT)
             if (oncenters)
             -> find_height_bounds(zGridCenter)

Author: hkershaw-brown

models/mpas_atm/model_mod.f90

@@ -4686,8 +4686,8 @@ subroutine convert_vert_distrib(state_handle, ens_size, location, obs_kind, ztyp
    fdata = 0.0_r8
    do i = 1, n
       where (istatus == 0)
-         fdata(i, :) = zGridFace(k_low(i, :),c(i))*(1.0_r8 - fract(i, :)) + &
-                       zGridFace(k_up (i, :),c(i))*fract(i, :)
+         fdata(i, :) = zGridCenter(k_low(i, :),c(i))*(1.0_r8 - fract(i, :)) + &
+                       zGridCenter(k_up (i, :),c(i))*fract(i, :)
       end where
    enddo