'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 : ndarray_1d
The first derivative of the loglikelihood function calculated as
the sum of `score_obs`'

def score(self, params, scale=None):

    return self.score_obs(params, scale=scale).sum(0)
