- add code that allows the use of ALLOW_OBCS?_CONTROL without exf/cal - fix initialisation (computation of diffrec) for obcs-ctrl - handle case of obcs?period == 0
- add code that allows the use of ALLOW_OBCS?_CONTROL without exf/cal
- fix initialisation (computation of diffrec) for obcs-ctrl
- handle case of obcs?period == 0

altMITgcm/MITgcm66h,altMITgcm/MITgcm66h,altMITgcm/MITgcm66h,altMITgcm/MITgcm66h,altMITgcm/MITgcm66h,altMITgcm/MITgcm66h,altMITgcm/MITgcm66h,altMITgcm/MITgcm66h