Clean up mk_GEOSldasRestarts and process_rst.csh #551

Revise processing of GEOSldas restarts:
- Adopt new GMAO_Shared utility `remap_restarts.py`.
- Re-mapping of LDASsa-formatted restarts is no longer supported.
- Update `ldas_setup` to python3.
- Replace `process_rst.csh` with cleaned-up python script `process_rst.py`.  
- Revised dummy restarts for CatchCN.
- Zero-diff except for: 
  - Treatment of restart variables needed by GCM but not LDAS (TSURF, WW, FR, CH, CM CQ); now remapped restart has same variables as source restart.
  - TSURF no longer has all-zero values.

src/Applications/LDAS_App/CMakeLists.txt

@@ -22,7 +22,7 @@ ecbuild_add_executable (
 set (scripts
    ldas_setup 
    process_hist.csh 
-   process_rst.csh 
+   process_rst.py 
    ens_forcing/average_ensemble_forcing.py 
    ens_forcing/ensemble_forc.py 
    ens_forcing/regrid_forc.csh