Check stomate output in sechiba history file

[luyssaert]

In slowproc section 4.2 with the title "Output the respiration terms and the net primary production (NPP) that are calculated in STOMATE" writes npp, resp_maint, resp_growth, ... to a sechiba history file. This is only when ok_stomate = y. If that is the case all those variables were just written to a stomate history file. Note that in slowproc the calculation of npp is wrong because it does not account for for atm_to_bm. Correct the way npp is calculated or even better delete this block of code.