Bounded and unbounded are not antonyms

It is currently possible to create a meaningless set of assumptions:

neither
```
    Dummy(zero=True, bounded=False) nor
    Dummy(zero=True, unbounded=True)
```
raise an error.

Further discussion is in the commit involving NaN.

- [ ] clean up S.in_Q discussion
- [x] change unbounded to infinite
- [ ] get good reference for definition of bounded (abs(x) < oo) and finite (0 < abs(x) < oo)
- [ ] what's the right answer for wester M24 test?
I provided weak references to the definition of finite meaning "something measurable; nonzero". This is also in accordance with the intent when we say "finite difference" -- you are using a nonzero difference to approximate a derivative. Nonetheless, since Python has already set a precedent for making finite include 0 I don't have a problem with sticking to that.

I don't know how to modify the assumptions without introducing something for "finite" so for now I have made it a private attribute, "_finite". It can be changed to nonzero_finite or anything else when we agree on a name.