Fix Time Formatting in NetCDF output

[larantt]

Hi devs!

I noticed a small bug in the time units that make them non-cf compliant and therefore unreadable by xarray.

The time units use all underscores as can be seen here:

netcdf hydro_2021-09-01_13_00_00 {
dimensions:
        LinkID = 6531 ;
        time = 288 ;
variables:
        int LinkID(LinkID) ;
                LinkID:long_name = "ID associated with each stream link" ;
        int time(time) ;
                time:long_name = "Time" ;
                time:units = "minutes since 2021-09-01_13_00_00" ;
                time:calendar = "julian" ;
        float outputs(time, LinkID) ;
                outputs:long_name = "Discharge" ;
                outputs:units = "m^3/s" ;
}

but xarray is unable to decode this properly and it drops the hours. I was able to fix it in xarray with the following line, but I was wondering if it would be easy to fix in the code?

ds['time'] = pd.to_timedelta(ds['time'].values, unit='m') + pd.Timestamp('2021-09-01 13:00:00')

Thanks for your help and for this awesome model!
Lara