Allow 'extrapolation' in synthetic control

[drbenvincent]

As highlighted in #255, the current synthetic control model is restricted to interpolation. That is, when the synthetic control is modelled as a weighted sum of untreated units, and that weighting sums to 1, then the model can only interpolate within the region of the control units.

We should consider adding the ability for synthetic control models to extrapolate, and that can be done by letting the sum of weights deviate from 1.

I have not yet thought about whether we should stick with the current WeightedSumFitter class and make it more customisable, or if we should create different classes. Maybe the former.