Have a uniform way when interfacing json files

[augustjohansson]

Right now there's

1. set/getJsonStructField
2. assignStructParams / assignJsonParams
3. updateJson
   All these have (probably) their pros and cons, but I'd suggests we'll use the set/getJsonStructField, and possibly modify these to handle the cases covered by 2 & 3. Then these can be deleted. Or we simply delete 2 & 3 now and fix errors as they appear.