Daniel Georg Holzfeind

@madsdk This MR provides several more math environments. Use `env` to get them. - example - theorem - lemma - corollary - proof (also callable via `proof`) - proposition -...

It is better to use the one line variant. So that the code remains clear.

When calling `open`, or `doc`, e.g., `doc rref` or `open rref` the octave kernel hangs up and have to be killed or restarted to work further on.