"comment"> Write online Lagrangian initial conditions file.

 init_lag(Mobj, type, file)

 DESCRIPTION:
   Create the Lagrangian initial positions netCDF file.

 INPUT:
   Mobj - MATLAB mesh object containing fields:
       have_bath - boolean flag indicating whether the bathymetry is
       loaded.
       native_coord - string of the native coordinate type.
       h - water depth on nodes.
       xc, yc or lonc, lat - element centre coordinates (cartesian or
       spherical depending on native_coord).
   