user should not be allowed to set dtime in user_nl_cam

The physics timestep and atmophsere/land coupling frequecy is set by the ATM_NCPL variable in env_run.xml.

However, if a user sets 'dtime' in user_nl_cam, it will take precedence over the value that would be set by ATM_NCPL. But the value used by the CLM will still be that determined by ATM_NCPL. This creates an invalid configuration that will run but product incorrect results.

Atmopshere should follow the lead of the land model and error out if the user tries to set dtime in user_nl_cam.