Renaming oracle attributes

[tbazin]

I find the name data to be confusing in the oracle's class attributes.
I'd rather go for symbols instead, to clearly distinguish it from the concrete data, the features. With an -s, because there are several possible symbols accessible for a given state!

And rename f_array to feature at the same time, because, who knows, a feature doesn't necessarily have to be an array! And anyway, the interface should be kept separate from the actual implementation, so mentioning arrays is bad practice.

[wangsix]

That makes sense. I will adjust it if you have not.

[tbazin]

I've just done it in the dev branch, I'll merge it with master in the coming days.
I actually went for symbol instead of symbols, since all transitions leading to state i are actually labeled
by the same symbol, so no need for multiple elements (my bad on this mistake).