CMIP6 activity_id can be a list of terms

[paolap]

Just noticed this in the documentation:
an activity_id identifying the responsible “MIP”. In a few cases it may be appropriate to include multiple activities in the activity_id (separated by single spaces). An example of this is “LUMIP AerChemMIP” for one of the CMIP6 land-use change experiments.

It's not urgent since currently they are only publishing 'CMIP' (DECK) output, but we'll need entually need to treat activity_id as a list!