Exact L-moments of several distributions

[jorenham]

Not part of the public API.
This will result in better L-moments for several scipy.stats distributions.
These distributions will also see a significant performance increase when fitted with l_fit.

The relevant distributions are:

• scipy.stats.uniform
• scipy.stats.logistic
• scipy.stats.expon
• scipy.stats.gumbel_r
• scipy.stats.genextreme
• scipy.stats.genpareto
• lmo.distributions.kumaraswamy
• lmo.distributions.wakeby
• lmo.distributions.genlambda

All other scipy.stats distributions will also see a performance increase in the l_fit, due to some arbitrary optimizations.

towards #5