CORE / TIER1 / TIER2 priorities

[jesusff]

We need to clarify the meaning of the priority labels internally for I4C. For CORDEX, the meaning is quite loose regarding how mandatory is it to produce a field, especially sub-daily data.

For the moment, in the I4C data request, both CORE and TIER1 fields are mandatory, the difference being that CORE variables will fit in a central server, while TIER1 ones might need to be stored locally and exchanged in another way. These are 1hr 3D variables only requested by the emulators task (T3.3) groups, which are mostly the CPRCM output producers.