PointEstimateMixin overwrites existing ancil data

[drewoldag]

Bug report
Calling PointEstimateMixin.calculate_point_estimates will overwrite existing ancil data. This is because we're using .set_ancil instead of .add_to_ancil.

The solution here would be something like:

if qp_dist.ancil is None:
    qp_dist.set_ancil(ancil_dict)
else:
    qp_dist.add_to_ancil(ancil_dict)