Main recipe vs. Atomic recipe: Make the format more general

Currently, there are some fields in the main recipe which need to be specified in a different way from the atomic recipe for the splitting to work correctly. For example:

# Atomic recipe
 Region:
    latmin: 20 # Mandatory, int: minimum latitude
    latmax: 80 # Mandatory, int: maximum latitude
    lonmin: -20 # Mandatory, int: minimum longitude
    lonmax: 40 # Mandatory, int: maximum longitude
# Main recipe
  Region:
   - {name: "UE", latmin: 20, latmax: 80, lonmin: -20, lonmax: 40}

Or,

# Atomic recipe
    System: # multiple systems for single model, split if Multimodel = F
      name: ECMWF-SEAS5.1
# Main recipe
    System: # multiple systems for single model, split if Multimodel = F
      - {name: ECMWF-SEAS5.1}

Ideally, the format should be uniform to avoid confusion for users.

FYI @nperez