Load metadata from coscine if not in cache

Currently, the actual metadata of a file object seems not to be loaded anywhere, except one forces it in this method. However, if the metadata is not already in the cache and I ask for it on the file object, I would expect it to be loaded from coscine. Therefore, I deleted the return None such that a loading is attempted.

A question on the cache: Currently, I do not see a method to easily populate the cache with the metadata in one directory. As such, the nice .dataframe() method only provides a more or less empty DataFrame - calling the dataframe should ideally result in a 'get all metadata into cache and load DataFrame with this data' request, IMHO.

src/coscine/object.py

@@ -309,7 +309,6 @@ class FileObject:
             if self._cached_metadata:
                 for key in self._cached_metadata:
                     return self._cached_metadata[key]
-            return None
         uri = self.client.uri("Tree", "Tree", self.resource.id)
         args = {"path": self.path}
         data = self.client.get(uri, params=args).json()