Cleanup camhub_comp as hub component

Ideal Engineering Time: 5 days
Some clean up to the camhub_comp module.

• Move the snowhlnd array from comsrf to camhub_comp srfflx_state. (done)
• Move subroutine srfxfer to camhub_comp as a method
• Move subroutine diag_surf from diagnostics.F90 into camhub_comp
• Change camhub_comp merge_srf routine, so it doesn't use update_srf_fluxes, but does one update all at once.
• Be careful about data that's in the camhub_comp derived types. Remove data not needed there, and add data that is needed (move rho, netsw, and psl, from ccsm_msg to here).