Updates incorporating/correcting issues #15 #17 and #18

The following changes have been made:

mip_era has been reset to 'CMIP6' (this reverses change made in #15)
CNRM-CM 6.1 has been changed to 'CNRM-CM61' to satisfy character restrictions and to be consistent with other model names (see issue #13)
tracking_id has been left as 'SNAPSI/.*' consistent with recommendation from Martin Juckes
allowable_model_components for each experiment have been set to more reasonable values (see #15)
The dimensions and cell_methods changes suggested by issue #17 have been made.
All files have been run through a json pretty-printer; dates have been updated.

cedadev / snap