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  - The symbolic data is given to allow real kernels to find a cell's
    depth / box.
  - The P2P did not compute the correct indices for the U-list items

This is done to make constructors consistent accross the algorithms.

The Lagrange and Chebyshev kernels are disabled until they are made
compatible with the StarPU algorithm.

 - 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.

Group some of the node symbolic data (depth, morton index) in a
symbolic_data_t object. This will allow easy manipulation.

Making the particle container part of a node make (as opposed to
dynamically allocated) speeds up tree creation as no allocation takes
place for empty nodes.

This feature is not well supported by compilers. The traits values are
only used at comiple time.

The split_to_height method splits all nodes in the tree until the given
height.

This constructor's aim was to provide a static_assert message that would
indicate the error. This could have hindered sfinae as the constructor
was seen as existing without failing compilation early.