Bug Fix: 'PHP Notice:  Undefined index' error.
Merge branch 'refs/heads/master' into develop
Added some basic unit tests.
Added a 'type' field in exports of Attorneys.
Added a payment date.
Added 'contact by post' and 'contact in Welsh' booleans to 'Correspondence'.
Changed how formatting $lpa->id works.

Rather than automatically formatting the id with the getter, it is now done by doing:
Opg\Lpa\DataModel\Lpa\Formatter::id($lpa->id)
Added docs and fixed typos.