Separate CDS request logic from other preprocessing

[trossi]

A lot of runtime in ModGP-run_prep.R is spent idling and waiting CDS to process requests. Other processing in the script requires more resources (e.g. memory), so it would be useful to separate the waiting/downloading logic from the actual processing.

This could be implemented as a separate initial script that does only the CDS requests and downloads all necessary Data/Environment/TEMP_* files that are then processed by ModGP-run_prep.R further as currently.

See related discussion in #25 and related issues #33 #35 #37.

[MichalTorma]

I think this is a good idea but not hight on a priority list. It's only a little wasteful - ideally data prep like this should be done outside LUMI completely - since it doesn't need much processing anyway.