User story: axes in 3D-plot

[hgeorgsch]

The student (normally) needs to see the axes, and values, independently of zoom factor.

(Now a grid is faintly visible only when the student zooms in.)

Ideally, this should be configurable so that the question editor can select the configuration of axes.

[hgeorgsch]

Two more extra options would be useful:

1. Choose boxing axes instead of axis cross, and
2. make surface plot opaque.
   If this is added, we can close this issue.