Fix expected type for BMI configuration

[elbeejay]

Was getting the following warning when running the run_pyDeltaRCM_as_BMI.py script:

Input for model_output__opt_time_interval not of the right type. model_output__opt_time_interval needs to be of type <class 'float'>

So the super minor commit in this PR just addresses that by changing the expected type for time to be 'int' instead of 'float' to match the convention in the non-BMI method. I don't see anything in the CSDMS BMI naming convention that suggests the "time_interval" variable must be a 'float'.

[ericbarefoot]

This looks good to me. I wonder if in the future, we could come up with a script that compares across the wrapper and the defaults to make sure that all parameters in the wrapper correspond to a real parameter in the model, and that the expected types match.