Introduce factories to create data group elements

[sprain]

• Use factories instead of setters in order to guarantee complete data group elements

[ppietak]

Good to see changes we talked about :)
Do we still need nullable fields in Address? Like building number? Maybe finally it will be merged with different field either way?

[sprain]

Do we still need nullable fields in Address? Like building number? Maybe finally it will be merged with different field either way?

IMO yes.
Not every address has a street.
Not every address with a street has a building number.

The purpose of StructuredAddress is to keep all these fields separate. This is how it's defined in the specs of the qr bill.

[sprain]

I'll merge this in order to make users aware of the upcoming major refactorings. Further improvements may still be done.