Support for interim numbers.

[Johannestegner]

Type of change

• New feature

Description

This PR includes a few different changes to make the package adhere to the v3.1 specification (https://github.com/personnummer/meta#package-specification-v31).

Interim numbers have been introduced to the package. Tests included.
A strlen check have been made on the initial input (in constructor), throwing an error if the string is under 10 chars and over 13 (if it is, it's not a correctly formatted number and can be discarded directly).

The public function getDate() : Date; implementation have not yet been added, hence, the package will not be fully ready for a 3.1 compliant release, while, if released, it should most likely be a major or minor release (seeing this is a lot more than a patch change).

Related issue

#60

Motivation

We want the packages to be using the latest spec! :)

Checklist

• I have read the CONTRIBUTING document.
• I have read and accepted the Code of conduct
• Tests passes.
• Style lints passes.
• Documentation of new public methods exists.
• New tests added which covers the added code.
• Documentation is updated.