Improve SphericalExpansionByPair

[Luthaf]

This calculator was added in #147, but could be improved and integrated more with the rest of the code:

• [ ] we would want to run the calculation over pairs in parallel
• [ ] if the step above makes it fast enough, it might be interesting to use an explicit pair by pair calculator then an explicit sum when we only have 1-4 systems in the spherical expansion.