code: def getsize(self, key=None):
          
          if key is None:
              return os.path.getsize(self.filename)
          return hdf5.ByteCounter.get_nbytes(
              h5py.File.__getitem__(self.hdf5, key))
docs: Return the size in byte of the output associated to the given key.
          If no key is given, returns the total size of all files.