Remove the `up` and `down` attribute pointers. They were used to be
able, in the future, to build a cell from separate parts. In the
meantime, they were made to point to local attributes `m_data` and
`l_data`.

Add setters and getters {set,get}{MultiopoleData,LocalExpansionData}.

Add setter and getters {set,get}Data{Up,Down} for leagecy purpose. These
should be deprecated and removed in the future.