- Whitespace cleaning

 - Divide the FCostCell inner data layout into two sub-types:
   multipole_t and local_expansion_t. Two class attribute are accessible
   through the `getMultipoledata` and `getLocalExpansionData` methods.
   Define the optional symbolic_data_t type to store the cost of each
   operator.

 - 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);

 - Update the balanced algorithm to use the new interface.