Fix negative values in the pairwise computations of SqMahalanobis

[devmotion]

Fixes negative values in the pairwise computations with SqMahalanobis, in the same way as done for the (weighted) (Sq)Euclidean distances (#161) and CosineDist.

On master:

julia> using Distances

julia> a = [0.3 0.3 + eps()]
1×2 Array{Float64,2}:
 0.3  0.3

julia> pairwise(SqMahalanobis(ones(1, 1)), a; dims = 2)
2×2 Array{Float64,2}:
  0.0          -2.77556e-17
 -2.77556e-17   0.0

julia> pairwise(SqMahalanobis(ones(1, 1)), a, a; dims = 2)
2×2 Array{Float64,2}:
  0.0          -2.77556e-17
 -2.77556e-17   0.0

With this PR:

julia> using Distances

julia> a = [0.3 0.3 + eps()]
1×2 Array{Float64,2}:
 0.3  0.3

julia> pairwise(SqMahalanobis(ones(1, 1)), a; dims = 2)
2×2 Array{Float64,2}:
 0.0  0.0
 0.0  0.0

julia> pairwise(SqMahalanobis(ones(1, 1)), a, a; dims = 2)
2×2 Array{Float64,2}:
 0.0  0.0
 0.0  0.0

[devmotion]

Test error https://travis-ci.org/github/JuliaStats/Distances.jl/jobs/731222246 seems to be unrelated

[dkarrasch]

Test error https://travis-ci.org/github/JuliaStats/Distances.jl/jobs/731222246 seems to be unrelated

True, but just to make sure...

[devmotion]

Tests pass now 🎉

[devmotion]

Is it possible to make a new release with this fix (I already updated the version number)? 🙂 Some tests in KernelFunctions are broken due to this problem.

[nalimilan]

Already done by @dkarrasch! JuliaRegistries/General#22239

[devmotion]

Oh great, somehow the comments to the commit didn't show up on my phone. Sorry for the noise!