Preparation for new timestepping scheme #2390.
Main changes:

1. Initial euler timestep now handled in stp and not in TRA/DYN routines.
2. Renaming of all timestep parameters. In summary, the namelist parameter is now rn_Dt and the current timestep is rDt (and rDt_ice, rDt_trc etc).
3. Renaming of a few miscellaneous parameters, eg. atfp -> rn_atfp (namelist parameter used everywhere) and rau0 -> rho0.

This version gives bit-comparable results to the previous version of the trunk.