[Easy] fix imagestack.write so that it correctly appends json suffix …

…to experiment.json (#450)

* make actions consistent with documentation.

starfish/stack.py

@@ -761,12 +761,12 @@ def tile_shape(self):
         return self._tile_shape
 
     def write(self, filepath: str, tile_opener=None) -> None:
-        """write the image tensor to disk
+        """write the image tensor to disk in spaceTx format
 
         Parameters
         ----------
         filepath : str
-            path + prefix for writing the image tensor
+            Path + prefix for the images and hybridization_images.json written by this function
         tile_opener : TODO ttung: doc me.
 
         """
@@ -823,6 +823,8 @@ def tile_opener(tileset_path, tile, ext):
                     ),
                     "wb")
 
+        if not filepath.endswith('.json'):
+            filepath += '.json'
         Writer.write_to_path(
             self._image_partition,
             filepath,