add fill value in output

**issue:** There is no fill value attribute in output netcdf

**resolution:**
Define in write_simout.f90
https://www.unidata.ucar.edu/software/netcdf/netcdf-4/newdocs/netcdf-f90/NF90_005fDEF_005fVAR_005fFILL.html#NF90_005fDEF_005fVAR_005fFILL

**caution:**

1. Be careful that missing values are propagated through routing process (runof read, remapping, delayed runoff, and then channel routing processes).

2. If there is missing runoff in hru at one time, possibly hru does not have any hydrologic model hru overlapped, meaning there is no valid runoff through the simulation period.

3. How would you handle for effect of missing hrus on downstream segment routed runoff.