- Add FMM operator overloads that take specific node symbolic data,
   multipole and local expansion instead of the whole node data.

 - Add a mutex to allow concurrent execution, this implies the explicit
   copy constructor/operator because the mutex cannot be copied/moved.

 - Add a fuse_results operation to gather the information from the
   kernel copies when a multi-threaded algorithms uses them (eg. tasks,
   StarPU).

 - Make the multipole and local expansion types explicit.