support time-invariant forcing perturbations

[aekiss]

It would be useful to have support time-invariant additive or multiplicative perturbations, so we don't necessarily need to specify the perturbation at every forcing timestep. This would be particularly handy for the IAF runs.

This could be implemented by applying the perturbation at every time if the perturbation file is missing a time axis.

[russfiedler]

@aekiss @nichannah I've made a start on this. First step is just to apply a constant scaling. I've got this in /scratch/v45/raf599/libaccessom2 . Just looks like a couple of changes to libcouple/src/field.F90 and atm/src/forcing.F90. Could you have a quick look to see if it's the sort of change that looks interesting and if I'm going about it the right way.

[aekiss]

thanks @russfiedler that looks good to me, I only had a few minor comments

[aekiss]

closing - this use case is supported by the new perturbation feature - see #30 and
https://github.com/COSIMA/access-om2/wiki/Tutorials#Scaling-the-forcing-fields