missing sref labels

[kohlhase]

sTeX does not add labels fir id= keys in \sref in some situations. This issue collects them:

• in \begin{spfstep} (and probably sproof and all others there as well)
• in \begin{sproblem} and \begin{assignment}
• in \begin{frame}: I think we need to be able to do \sref on these too. This currently is the only place, were we cannot. Maybe we need to go towards \begin{sframe} and patch that into \begin{frame} to be consistent with the statements.

We also need to think about referencing the excursions. Here we have to thread the file= argument through to the internal \sref call in \excursionref.

[Jazzpirate]

spf*, sproblem and assignment are done.

I disagree with frame/sframe: I don't think it's a good idea to introduce another environment for that, and the solution is as simple as doing \sreflabel in the frame ;)

Re: excursions: I don't yet understand the problem well enough to have an opinion