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.