Quentin 6/11/2018: coupling_naturen.F90, correction: wrong time variable in...

Quentin 6/11/2018: coupling_naturen.F90, correction: wrong time variable in call coupling_tsz0  (PTIMEC --> PTIME). PTIME is the time in seconds since UTC midnight necessary for applying surface temperature or sensible/heat flux forcings at the right time (especially if MNH uses restart)