In ELM `Flrr_flood` should only be accounted for when two-way land-river coupling is turned on

[Gautam Bisht]

Currently, the flux of flood water from MOSART (i.e. Flrr_flood) is being added even when the two-way land-river coupling is not turned on. Technically, the Flrr_flood is supposed to be 0.0_r8 for the default run (i.e. one-way land-river coupling). However, @tanzeli1982 found out that when MOSART was blowing up due to numerical instability because the MOSART time step was too large, Flrr_flood was not 0.0_r8. The fix is to only add Flrr_flood in ELM when the two-way land-river coupling is turned on.

cc: @tanzeli1982 , @liho745, @hydrotian