decide how to initialize l2x coupler fields in CESM driver

[dgergel]

Previously, with VIC 4.0.4 in RASM, we initialized the l2x coupler fields in the l2x_data_struct with 0s instead of SHR_CONST_SPVAL (which is 10e30). We do not populate all of these fields during model initialization, after which we pass them to WRF before running for a timestep. We need to decide whether we want to set fields to 0 or SHR_CONST_SPVAL in VIC 5 in RASM. The danger we run into if we set fields to 0 is that we will end up not catching fields that aren't getting populated when they should be.