Set stop time flag in WRF-CTSM coupling

A stop_flag denoting the last timestep of WRF-CTSM coupling should be passed to CTSM from WRF.

Please note that the method used for this issue is different from issue #909 (sending the restart flag) and needs a bit more work.
Within the call to CTSM, the WRF stop time flag would always be false if we are calling from the physics scheme, as we are always at least a time step before being at a stop time.

For this step, we should get the current time string and compare it with stop time string to determine if this is the last time step or not.