Merge local changes into KNMI-OSS version

Merged Alexander van Heukelum (TriOpSys) requested to merge (removed):master into master

Changes in use of CMIP6 data (data is accessible read-only). Make links in the data directory structure. Revert the CMIP6 Tglobal recalculation. Check if/how historic data needs to be grafted into the data.