Audit databraid

[jeanluct]

I learned a fair bit in writing annbraid (#99). One thing is that we need to check more carefully the methods from braid that should apply to a databraid. Doublecheck that everything is ok. At a bare minimum, I think we should deny access to cycle, since this is another function that only makes sense for iterated closed braids.

Same thing about the constructor.

[jeanluct]

Ok, removed a couple of methods. conjtest doesn't sense since it can't possibly work with databraids unless everything commutes, which is not interesting. cycle requires iteration.

I'm on the fence about tntype. In some sense it does require iteration. Leave it in for now.