Loglikelihood function for Gamma exponential family distribution.
Parameters : | Y : array-like
mu : array-like
scale : float, optional
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Returns : | llf : float
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Notes
llf = -1/scale * sum(Y/mu + log(mu) + (scale-1)*log(Y) + log(scale) + scale*gammaln(1/scale)) where gammaln is the log gamma function.