Configurable defaults (groups) for serialization

[akomm]

Related to this.

Basically if you want to hide information by default and explicitly provide it via group, you are doomed to take care of setting 'Default' group every time you serialize data. This might be a potential security problem.

My proposal is to provide a defaults configuration, including the groups as a starter (any other defaults useful?)

[goetas]

closing this for the same reasons emerged in #427

(#491 need to be verified)