Only call marbl_comp_nhx_surface_emis if lcompute_nhx_surface_emis ==
.true.. Also, set all components of marbl_surface_forcing_internal_type
to 0 at construction (if lcompute_nhx_surface_emis == .false. then
nhx_surface_emis is never set; the same may be true for other components
as well).

All POP tests run with lcompute_nhx_surface_emis = .true., so this
should not change answers in our test suite... but I think we should add
a lcompute_nhx_surface_emis = .false. test to POP.