Remove CompositeCellClass template argument.

Add SymbolCellClass, PoleCellClass and LocalCellClass to
FGroupOfCellsDyn and FGroupTreeDyn.

Remove methods getCompleteCell, getUpCell and getDownCell. It is not
possible to recreate a cell anymore.

They are replaced with getSymbolic, getMultipole and
getLocalExpansion. Those new methods take the same parameter as the
previous ones: the required cell index in the group of cells.

The forEachCell method callback signatures are changed from
    void(CompositeCellClass, Args...)
to
    void(SymbolCellClass*, PoleCellClass*, LocalCellClass*, Args...)
in order to take into account the fact that cells are not recreated
anymore.