generate_sourcesink_nc.py
The script will combine two sources (NWM + sflux_prc) and write vsource/msource/vsink  to one netcdf file.

Required inputs:
1. path to the run directory:
    rundir = "/sciclone/schism10/lcui01/schism20/ICOGS/ICOGS2D/GA_RUN02"

2. model start time:
    model_start_time_str = ""2018-08-01 00:00:00"

The script assumes the following files are in the rundir/:
    hgrid.gr3
    
    source_sink.in   # These four 
    vsource.th
    vsink.th
    msource.th 

    sflux/flux_prc_1.****.nc (precipitation will be interpolated to elements and converted to volume source)
   
Finally, the script will write one netcdf file into rundir/Orignial+sflux_source_sink/source.nc, which looks like this:
netcdf source {
dimensions:
	nsources = 1283607 ;
	nsinks = 34 ;
	ntracers = 2 ;
	time_msource = 2 ;
	time_vsource = 1489 ;
	time_vsink = 1489 ;
	one = 1 ;
variables:
	int source_elem(nsources) ;
	float vsource(time_vsource, nsources) ;
	float msource(time_msource, ntracers, nsources) ;
	int sink_elem(nsinks) ;
	float vsink(time_vsink, nsinks) ;
	float time_step_vsource(one) ;
	float time_step_msource(one) ;
	float time_step_vsink(one) ;
}
