read from file instead of explicit instantiation

Most definitions can stay the same, but the sets inside `apps` and `slots` need to change from
```prolog
apps([a,b,c]).
slots([x,y,z]).
```
to something that reads input from a file

### random rankings

For testing, the following predicate generates random rankings:
```prolog
rprefs(Goal,X) :- inv(Goal,Other),agent(Other,L),random_permutation(L,X).
```