Merge branch '20-check_tfrecords_files' into 'develop'

Resolve "Add warning message when no output_shape or output_type is missing"

Closes #20

See merge request remi.cresson/otbtf!56

otbtf/tfrecords.py

@@ -172,6 +172,9 @@ def read(self, batch_size, target_keys, n_workers=1, drop_remainder=True, shuffl
         :param num_parallel_calls: number of parallel calls for the parsing + preprocessing step
         :param kwargs: some keywords arguments for preprocessing_fn
         """
+        for dic, file in zip([self.output_types, self.output_shapes], [self.output_types_file, self.output_shapes_file]):
+            assert dic, f"The file {file} is missing!"
+
         options = tf.data.Options()
         if shuffle_buffer_size:
             options.experimental_deterministic = False  # disable order, increase speed