Implement a SkewLogistic distribution

[fritzo]

Addresses #2868

This is another smooth asymmetric distribution with asymptotically exponential tails addressing #2868 but hopefully more numerically stable than SoftAsymmetricLaplace #2872 which is waiting on torch.special.logerfc() pytorch/pytorch#31945.

Tested

• added to conftest (thank goodness for gof tests!)
• confirmed numerical stability in a large real-world model

[fehiepsi]

LGTM! The implementation shows that this is like a logistic distribution, only different in that the base distribution is not Uniform(0, 1), but Uniform(0, 1)^(1/assymetry), which implies the skewness.