Support Cases-Environment

[AndreG-P]

Still not supported are \begin{cases}. We should be able to translate them already. For example:

{z \choose n} = \begin{cases} \frac{\Gamma(z+1)}{\Gamma(n+1)\Gamma(z-n+1)} & n \geq 0 \\ 0 & n < 0 \end{cases}