Tabulated pair functions calculator

[Luthaf]

We need some calculator that can take in one or multiple splines, and compute a per atom/per structure functions summed over neighbors.

$$ < n | f > = \sum_j f(r_{ij}) $$

This would enable multiple things:

• calculation of pair potential as a baseline potential (without requiring an external software)
• calculation of basic pair descriptor (the G2 functions for BPSF would be a possibility)