Reformat private properties & method parameters.

From snake case to lower camelcase.
Make `PostnrValidate` methods return the input.

This simplifies the usage of these functions.
Rename `NyPostnr::gyldigFraDato` to `dato`.

Update tests accordingly.
Rename `NyPostnr` to `PostnrNy`.

[Update tests]
Rename `PostnrEndring` methods' adjectives.

Use masculine/feminine adjectives, unifying the interface.
Use new adjective forms in `Register` methods.
[Update tests]

Formatting.