Code and documentation to produce subseasonal climate briefing forecast of prlr and tas

Hi @jcos

I have included the instructions to create the post-processed outputs for the climate briefings for the subseasonal forecast.

Could you please try to use it and let us know if it works?

I have marked this merge request as a draft because it requires !264 (merged) to be merged first (which produces the seasonal products and it was already tested by you).

Thanks,

Núria