Michel group polish - Part 2

@atnnn I did a little fix in `_scripts/dataexplorer.py`. In some cases, I forgot to reset some flags.
I tested the result, and it works fine now.

All the rest is small fixes (like picking the appropriate language to highlight example, adding the missing `io` fields, adding the missing max/min/sum/avg in the index files etc.)

I also removed the examples for `group`/`ungroup` from the files `index.md` for JavaScript because it was making docs too big for the data explorer.

@atnnn -- could you do a quick review for me?