Primitives in geometry/curves

Since there is now a distinction between the representation of a primitive and the recognition algorithm, segment computers have been renamed. 

It remains to 
- [ ] implements an arithmetical DSS with a,b,mu
- [ ] implements an arithmetical DSS with a continued fraction for a/b
- [ ] implements a standard DSS in 3d with three 2d DSS
- [ ] implements a stabbing line with its preimage
- [ ] implements a stabbing circle with a reference circle
- [ ] implements a one-balanced word