LILAC: Consider adding capability for atmosphere to dictate which fields it will want

In implementing the initial WRF-CTSM coupling, it looks like there are a lot of fields that are exported by CTSM that are not needed by WRF. We should consider adding the capability for the atmosphere to set, in initialization, which fields it will want. If it doesn't need a particular field, we can avoid adding that field in the coupling, which could improve performance.

This may only be worth doing if the performance gain is significant.