score, first derivative of the loglikelihood function
Parameters:
params (ndarray) – parameter at which score is evaluated
scale (None or float) – If scale is None, then the default scale will be calculated.
Default scale is defined by self.scaletype and set in fit.
If scale is not None, then it is used as a fixed scale.
Returns:
score – The first derivative of the loglikelihood function calculated as
the sum of score_obs