automata-subclasses naming mess

Very related to HOAF issue#63.

Currently we have spot::is_deterministic() to detect universal automata, and spot::twa::is_alternating() to detect non-existential automata.

I would suggest we quickly deprecate is_alternating() (introduced in Spot 2.3) and introduce the opposite is_existential().

It would make more sense to rename is_deterministic() as spot::is_universal(), and redefine is_deterministic() as aut->is_existential() && is_universal(aut).

Also the fact that is_existential() is a method of twa (because it is cheap), and is_universal is a function (because it is costly) is unfortunate. Let us define both at the same place for symmetry.