Weibull mode (#2804)

* Add mode to Weibull

* Line breaks, removed "self.parameter" from expressions.

Author: ericmjl

pymc3/distributions/continuous.py

@@ -1554,6 +1554,9 @@ def __init__(self, alpha, beta, *args, **kwargs):
         self.median = beta * tt.exp(gammaln(tt.log(2)))**(1. / alpha)
         self.variance = (beta**2) * \
             tt.exp(gammaln(1 + 2. / alpha - self.mean**2))
+        self.mode = tt.switch(alpha >= 1, 
+                              beta * ((alpha - 1)/alpha) ** (1 / alpha), 
+                              0)  # Reference: https://en.wikipedia.org/wiki/Weibull_distribution
 
         assert_negative_support(alpha, 'alpha', 'Weibull')
         assert_negative_support(beta, 'beta', 'Weibull')