Behler-related implementation details

[mastricker]

Writing an issue for myself as a reminder to check these things: For gradients:

• store intermediate results without summation (needed for kernel partial derivatives)

Behler Angular symmetry functions, implementation questions:

• precomputation of distances^2
• precomputation of cos_theta (not only the angle, but directly the cos)
• prefactor 2^(1-zeta) multiplication outside?
• n2p2 uses r_ij = r_i - r_j (!) for distances
• derivatives for angular narrow: 3x cos_theta
• usage of collinear/non-collinear derivatives
• SymmetryFunction::Order switch for Angular1/2: different number of arguments in function?
• Calculation of triplet-cutoffs where in BehlerFeature or in Calculator of some sort?
• compute_helper of cutoff function: distinction between PairOrder and TripletOrder
• same interface for angular wide/narrow, argument list is one more for the narrow - the distance between the j and k atom.