Incorrect unit for RAIN_RATE ouput

[lcunha0118]

The output defined as RAIN_RATE in output_var_names has unit of m ( see [https://github.com/NOAA-OWP/cfe/blob/39c18a100dcb5c664900711c8f65f786642f0671/src/bmi_cfe.c#L203]) according to the bmi functions, but the output actually is in mm.

Current behavior

Incorrect unit (mm) for RAIN_RATE output.

Expected behavior

RAIN_RATE in m

Suggested fix:

Since all other variables are in m, we probably want to keep this variable in m, and convert the variable from mm to m within the code.