Fix default output name for NASA source (fixes #14)

When using --data-source NASA without explicit -o, the CLI defaulted to
elevation.tif (GTiff format). But the NASA code path writes raw binary +
.rsc sidecar, so GDAL couldn't open the file during EGM-to-WGS84
conversion (it expected a real GeoTIFF).

Fix: force output_format to ENVI for NASA sources in the CLI, which
makes the default output name elevation.dem. Also assert ENVI format
in dem.main() as a safety net.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: scottstanie

sardem/cli.py

@@ -202,6 +202,10 @@ def cli():
     else:
         bbox = None
 
+    # NASA sources write raw binary + .rsc sidecar, not GeoTIFF
+    if args.data_source in ("NASA", "NASA_WATER"):
+        args.output_format = "ENVI"
+
     if not args.output:
         if args.data_source == "NASA_WATER":
             output = "watermask.flg"

sardem/dem.py

@@ -408,11 +408,10 @@ def main(
         return
 
     # If using SRTM, download tiles manually and stitch
-    if output_format != "ENVI":
-        logger.warning(
-            "NASA data source only supports ENVI format. Ignoring output_format=%s",
-            output_format,
-        )
+    assert output_format == "ENVI", (
+        f"NASA data source only supports ENVI format, got {output_format!r}."
+        " Use COP or 3DEP for GTiff output."
+    )
 
     # Check for dateline crossing
     bboxes = utils.check_dateline(bbox)