Use multithreading for neighbor search on CPU

[Luthaf]

Built on top of #170 which will need to be merged first.

Some basic benchmarks on M1 max CPU

===========  CUTOFF=3  =============
500 atoms
   vesin with 1 threads took 0.600 ms
   vesin with 2 threads took 0.465 ms
   vesin with 4 threads took 0.363 ms
   vesin with 8 threads took 0.297 ms
   vesin with 10 threads took 0.307 ms

50000 atoms
   vesin with 1 threads took 54.110 ms
   vesin with 2 threads took 39.086 ms
   vesin with 4 threads took 36.121 ms
   vesin with 8 threads took 22.372 ms
   vesin with 10 threads took 23.723 ms

===========  CUTOFF=6  =============
500 atoms
   vesin with 1 threads took 4.655 ms
   vesin with 2 threads took 3.228 ms
   vesin with 4 threads took 2.840 ms
   vesin with 8 threads took 2.584 ms
   vesin with 10 threads took 2.461 ms

50000 atoms
   vesin with 1 threads took 368.325 ms
   vesin with 2 threads took 243.785 ms
   vesin with 4 threads took 173.398 ms
   vesin with 8 threads took 148.175 ms
   vesin with 10 threads took 129.841 ms

===========  CUTOFF=12  =============
500 atoms
   vesin with 1 threads took 31.788 ms
   vesin with 2 threads took 32.329 ms
   vesin with 4 threads took 32.312 ms
   vesin with 8 threads took 32.229 ms
   vesin with 10 threads took 31.437 ms

[Luthaf]

Benchmarks on Threadripper Pro 5945WX

===========  CUTOFF=3  =============
500 atoms
   vesin with 1 threads took 0.648 ms
   vesin with 2 threads took 0.663 ms
   vesin with 4 threads took 0.628 ms
   vesin with 8 threads took 0.537 ms
   vesin with 16 threads took 0.752 ms
   vesin with 24 threads took 1.066 ms

50000 atoms
   vesin with 1 threads took 61.002 ms
   vesin with 2 threads took 39.340 ms
   vesin with 4 threads took 42.001 ms
   vesin with 8 threads took 34.027 ms
   vesin with 16 threads took 28.520 ms
   vesin with 24 threads took 29.237 ms

===========  CUTOFF=6  =============
500 atoms
   vesin with 1 threads took 4.770 ms
   vesin with 2 threads took 3.427 ms
   vesin with 4 threads took 2.845 ms
   vesin with 8 threads took 1.600 ms
   vesin with 16 threads took 1.625 ms
   vesin with 24 threads took 1.929 ms

50000 atoms
   vesin with 1 threads took 416.281 ms
   vesin with 2 threads took 326.805 ms
   vesin with 4 threads took 259.598 ms
   vesin with 8 threads took 211.670 ms
   vesin with 16 threads took 219.905 ms
   vesin with 24 threads took 286.614 ms

===========  CUTOFF=12  =============
500 atoms
   vesin with 1 threads took 36.186 ms
   vesin with 2 threads took 36.180 ms
   vesin with 4 threads took 36.201 ms
   vesin with 8 threads took 36.130 ms
   vesin with 16 threads took 36.294 ms
   vesin with 24 threads took 35.981 ms