Fixups for examples related to `repeatergrid`

[Krastanov]

On preparing for merging #120 this was unfinished (better to fix it in a separate PR though):

• [ ] repeatergrid.md is not included in the rendered documentation (from the make.jl file)
• [ ] the webgl repeatergrid example does not start (the run button reacts, but no simulation runs)
• [ ] the gl repeatergrid example does not use its sliders -- the easiests solution is probably to just remove the sliders (and some easy-to-edit global variables should be declared instead of them)

The above are bugs that should be fixed. A "nice to have" would be to also DRY-out the code -- the four example files are very repetitive. That is not a priority though.