Add max precision configuration parameter. Closes #2372

This sets the limit at 6 by default but allows for user to increase it in advanced settings if needed.
The purpose of `aggParam.deserialize()` was to take the value from the db and convert it to the needed value. I'm not opposed to an init function, but I think it's important to realize that `aggConfig.fillDefaults()` is called often and `init()` sort of implies that it will run once at the beginning of usage