Support explicit infinity parsing from strings

[xmjw]

When we call I.to_string with an infinity we render (123,inf), but we cannot parse that back in 😔. This change allows us to parse this back into an interval.

For example:

>>> import intervals as I
>>> I.__version__
'1.5.5'
>>> ival = I.open(50,I.inf)
>>> ival
(50,+inf)
>>> istr = I.to_string(ival)
>>> istr
'(50,+inf)'
>>> inew = I.from_string(istr, conv=int)
>>> inew
(50,+inf)
>>> inew == ival
True

Hope this looks okay? I'm not especially skilled-up on Python...

[AlexandreDecan]

Thanks for submitting! I'll have a look asap!

[AlexandreDecan]

As a temporary workaround, the desired behaviour can be obtained by providing an appropriate conv function (e.g. the conv_with_infinity your provide in this pull request).

I'll add support for custom string representation/parsing for (positive/negative) infinity to from_string and to_string, probably by adding two new parameters pinf et ninf that could be used to specify the representation to use/parse for infinities. I hope to release this change tomorrow.

[AlexandreDecan]

I updated the two functions with pinf and ninf parameters. These can be used to specify which is the representation of a positive/negative infinity.

I also updated the documentation to add an example using +oo and -oo.
Release 1.6.0 will be on PyPi in a few minutes (I'm waiting for Travis to test the library on all Python versions first).

[xmjw]

Awesome - I've been using that workaround actually, I thought I should offer it back. Thanks for doing that, it's super helpful and cleans up my code a little.

[AlexandreDecan]

You're welcome ;) I'm glad to see someone is using this library ;-)