Bugfix/ensure maximum timestep stored in restart

• Add code_dt_max to mirror code_dt_min in gs2_time.

  Also adds save_dt_max to set the maximum time step.

• Use code_dt_max rather than dt0 when determining new time step.

  More closely mirrors treatment of minimum timestep.

• Store maximum timestep in restart files

• Restore maximum time step when initialising time step from restart file

Should fix issue #127

With this change the maximum time step (input variable dt0) is stored in the restart file and is restored when delt_option = ‘check_restart’. The value used can still be overridden by setting dt0.

This breaks backwards compatibility with existing restart files so should not go into master. If delt_max is not found in the restart file then we will fail to set the maximum time step or the time step history from the restart file.