FAdaptiveSequential: Make direct computation work with existing kernels
For a given leaf A, the existing symmetric kernels expect to receive the list of adjacent B leaves sorted by offset index. The offset index is computed by `compute_box_offset_index`. For direct computation, it ranges from 0 to 3^dim. A node's U list is not sorted. It is first looped over to compute the indices and stored in a sorted std::multimap. The map is then looped over to create the adequate arrays (managed through std::vector objects) to give to the kernel P2P call.
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