Double check correctness of causal impact calculation in scikit-learn flavour of DifferenceInDifferences

[drbenvincent]

At the moment we have a TODO comment which needs some attention

# This is the coefficient on the interaction term
# TODO: THIS IS NOT YET CORRECT ?????
self.causal_impact = (
    self.y_pred_treatment[1] - self.y_pred_counterfactual[0]
)[0]

From memory, this is not wrong, but achieved the right effect in a different manner to the pymc flavour of difference in differences.

Either way, we need to double check and resolve.