Fix JSON and JSONTIME export with gprints or rules

Fix JSON and JSONTIME export with gprints or rules

When you have gprints or rules, json is not formated correctly : a comma is inserted after each block, making two mistakes :
- there is no comma between "legend" and "gprints" block
- there is a trailing comma in "meta" block

I decided to rewrite this part and add the comma before each block, so this works in all cases : when you don't have any block, only one of them or both.