Weighted versions of properties

[DominikPeters]

The current versions of the properties are not right in cases where voters have weights (where for priceability, a voter's initial budget should be proportional to her weight, and for EJR and the like, the coalition needs to have total weight at least ell * total_weight/k). The algorithms should be generalized.

For writing tests, it would be useful to give the Profile class a method named something like convert_to_weighted(), which would combine several voters with the same approval set into a single voter with an appropriate weight. Then in tests, we can test for various instances that check_property(profile, committee) == check_property(profile.convert_to_weighted(), committee). (Alternatively one could keep things immutable and have a function that returns a copy of the profile with voters collapsed.)

[martinlackner]

Temporary fix: all relevant methods check whether the profile has unit weights (and throw a NotImplementedError otherwise)

[martinlackner]

I have added convert_to_weighted and convert_to_unit_weights methods for Profile.

[martinlackner]

Implemented by PR #68