Fixed error in list reader that would create intrahelical connections between double- and single-stranded segments; now using sscrossover connections

Improve how mrdna generates bead models; use `model.generate_bead_model(..., version=1.0)` for legacy behavior

Prevent PointParticle (or other Child objects) from looking up explicitly excluded methods such as __hash__ and __copy__ from ParticleType; Add ParticleType __copy__ and __deepcopy__ methods that return  to ensure types do not change upon copying a group or model

Pinyili2

See merge request !1