[Persistence_representations - Persistence_heat_maps] Simplify distance_from_diagonal_scaling

[VincentRouvreau]

distance_from_diagonal_scaling and squared_distance_from_diagonal_scaling have complicated implementations with pow, sqrt, etc (all missing std::). As far as I can tell, up to a multiplicative constant (which we can probably omit), the first one should be just std::abs(p.second-p.first) and the second one the square of p.second-p.first.

[mglisse]

It actually computes the projection on the diagonal (b+d)/2, and then the squared distance to that. It happens to be equal to |d-b|/sqrt(2). When computing the distance to the diagonal, Manhattan, Chebyshev and Euclidean are all proportional.