Exceptional float values

[IanBriggs]

Currently gelpia will crash if nan or inf are used.
They are recognized as variable names leading to an invalid parse.

What should we do with these inputs?

It might be reasonable to accept infinite numbers.

Nan, on the other hand, I think should be treated as improper input and a descriptive error returned.

[monadius]

I think that infinite bounds should be allowed for input variables. It is also important to be able to split infinite intervals properly. But I think everything should work without any modifications because Gaol implements a good midpoint function.