- Divide the FSphericalCell inner data layout into two sub-types:
   multipole_t and local_expansion_t. As the multipole and the local
   expansion layout is the same, they share a common implementation.
   Two class attribute are accessible through the `getMultipoledata` and
   `getLocalExpansionData` methods.

 - Change FMM operators signature to take advantage of the new
   layout. The operators only take as parameters the data they may need
   instead of the whole cell. For instance (simplified):

   void M2M(CellClass* parent, CellClass** children);

   becomes

   void M2M(multipole_t*  parent_m,    symbolic_data_t* parent_s,
            multipole_t** children_ms, symbolic_data_t* children_ss);