Fix ISO value handling

- #370 was caused by an ISO
  value being returned as a tuple (50,50). Check if we have a tuple and,
  if so, take only the first element.

src/picframe/get_image_meta.py

@@ -192,6 +192,9 @@ def get_exif(self, key):
                 for iso in iso_keys:
                     val = self.__get_if_exist(iso)
                     if val:
+                        # If ISO is returned as a tuple, take the first element
+                        if type(val) is tuple:
+                            val = val[0]
                         break
             else:
                 val = self.__get_if_exist(key)