Tiers and ensemble sizes

[martinjuckes]

land-ssp126, land-ssp585: corrected to tier 1 (1 member) + tier 2 (2 additional)
G7cirrus --> tier 2

There is a discrepancy between ES-DOC and CMIP6 CVs in the following (CV tier last):
pa-piAntSIC: 3 -- 2
piControl-spinup: 1 -- 2
esm-piControl-spinup: 1 -- 2

ssp245, ssp370: multi-ensemble in request, not in ES-DOC;