Add contants for handling infinities

[DXsmiley]

Operations such as tan(pi / 2) produce a value, zoo, which is something from Sympy.

• [x] Investigate what this value is.
• [x] Add a constant value to the calculator to access it easily.
• [x] Give it a better representation.
• [ ] Add it to the user documentation.

[Ytrog]

Maybe use the unicode ∞?

[DXsmiley]

At the moment, the calculator accepts either infinity or ∞ as input and uses the symbol for output. There's also the value complex_infinity for which no symbol is used. I'm not sure if such a symbol exists.

[Ytrog]

Apparently it is a lemniscate with a tilde on top: http://mathworld.wolfram.com/ComplexInfinity.html

http://functions.wolfram.com/Constants/ComplexInfinity/introductions/Symbols/ShowAll.html

Alas it is not represented in unicode: https://en.wikibooks.org/wiki/Unicode/List_of_useful_symbols#Mathematical