Add NISAR loading and auxiliary stack support

src/mintpy/load_data.py

@@ -637,10 +637,26 @@ def prepare_metadata(iDict):
         gunw_files = iDict['mintpy.load.unwFile']
         water_mask = iDict['mintpy.load.waterMaskFile']
 
+        if str(dem_file).lower() in ['auto', 'none', 'no', '']:
+            raise ValueError(
+                'mintpy.load.demFile is required for processor=nisar. '
+                'Please set it to a real DEM path in the template.'
+            )
+        dem_file = os.path.expanduser(str(dem_file))
+        if not os.path.isfile(dem_file):
+            raise FileNotFoundError(
+                f'No DEM file found for mintpy.load.demFile: {dem_file}'
+            )
+
+        if len(glob.glob(str(gunw_files))) == 0:
+            raise FileNotFoundError(
+                f'No input GUNW files found for mintpy.load.unwFile: {gunw_files}'
+            )
+
         # run prep_*.py
         iargs = ['-i', gunw_files, '-d', dem_file]
 
-        if os.path.exists(water_mask):
+        if str(water_mask).lower() not in ['auto', 'none', 'no', ''] and os.path.exists(water_mask):
             iargs = iargs + ['--mask', water_mask]
 
         if iDict['mintpy.subset.yx']: