change separator in GridDetails' names to :

Dear Laurent,

as discussed a few hours ago this PR changes the separator in the names of the GridDetails list to `:` to improve readability by humans and machine.

```s
# old style (nsd.ppm)
"4.5.20"
# new style (nsd:ppm)
"4.5:20"
```
This change should be safe because AFAIK the names are not used yet (only for printing in the report).

Best wishes,

Sebastian