adds goes-position auto-detection for G18

gridded_glm/libexec/_minute_gridder.py

@@ -112,6 +112,8 @@ def get_goes_position(filenames):
         return "east"
     if all("_G17_" in f for f in filenames):
         return "west"
+    if all("_G18_" in f for f in filenames):
+        return "west"
 
     # we require that all files are from the same sensor and raise an exception if not
     raise ValueError("could not determine GOES position - did you provide a mix of satellites?")