Planck (discrete exponential) Distribution

docs/tutorial:stats:discrete_planck

Named Planck because of its relationship to the black-body problem he solved.

M (t) = \frac{1 - e ^{- λ}}{1 - e ^{t - λ}}

h [X] = \frac{λ e ^{- λ}}{1 - e ^{- λ}} - lo g (1 - e^{- λ})

Implementation: scipy.stats.planck