Fix the zgap script

[fingolfin]

The zgap python script comes from HPC-GAP, where it resides in the bin dir, right next to gap.sh. The copy I added to this repository was not changed compared to that, and thus does not work correctly. This should be changed.

This will also require adapting the code in gap/znode.g, which I also just copied over. Note that this code uses ENTER_NAMESPACE directly, which seems like quite a hack -- I think we should rewrite it.

[fingolfin]

So one can try to get zgap working, by starting it from a dir containing bin/gap.sh, or (as I did) symlinking bin/gap.sh to the real gap.sh.
This leads eventually to this (unsurprising) error:

Error, the library file 'znode.g' must exist and be readable at /Users/mhorn/Projekte/GAP/gap.github/lib/init.g:324 called from
<function "ReadLib">( <arguments> )
 called from read-eval loop at /var/folders/d_/1zss2fnd6xdgclqnj8jj_27m0000gp/T/tmpJVw4FK/setup00.g:2
you can 'quit;' to quit to outer loop, or
you can 'return;' to continue
brk>

[fingolfin]

Alternatively, one could also remove the zgap script and the corresponding zgap chapter, as well as its API.