sanitize more possible input schemata of frac

[julianuu]

This is not polished yet.

It sanitizes the cases in https://github.com/ftilmann/latexdiff/issues/50 and more, such as

\documentclass{article}
\begin{document}
\begin{equation}
x = \frac \lambda b
\end{equation}
\begin{equation}
x = \frac12
\end{equation}
\end{document}

with spaces added and removed at will.

It still gives warnings because it tries to print capture group variables which haven't captured anything. It also does not deal with newlines or comment symbols yet. I am not sure what is the best way to fix either of these issues.

[julianuu]

now it also works for comment symbols with newlines

[julianuu]

Now I am happy with it.

[ftilmann]

Sorry for delay in dealing with this.