UGGrid should not query size of the message from the data handle on the receiving side (when loadbalancing)

According to a discussion on the mailing list there situations where this is not possible. Yet when scattering UGGrid asks the data handle for the size of the data sent for each element. Later on this size is passed to the scatter method. The call occurs in dune/grid/uggrid/uglbgatherscatter.hh#L99. This only happens for the communication with the datahandle when loadbalancing. All other communication seems to use dune/grid/uggrid/ugmessagebuffer.hh#L83-L107 which does the right thing.