UserStatus and dateTo might be optional

See merge request !57

Om 41 ansattnr

See merge request !53

UBWs documentation says that UserStatus is optional. Rumours say that
dateTo is optional too!

Adjust Firma models

See merge request !56

The API has entries without these fields, which has to mean that they
are optional.

Om 36 avgiftskoder

See merge request !55

Add methods for koststeder endpoint

See merge request !51

The value now matches the fixture we use. Since request_mock returns
what we want it doesn't really matter, but aesthetically it makes more
sense.

Add ext_arch_ref_col to SaveTransactions arguments

See merge request !52

This variable can be used to store an external reference to an
transaction.

Add get_anlegg method to client

See merge request !50

Add Kontoplan model for get_kontoplan method

See merge request !49

Achieves the same thing as get_begrepsverdier(company_id, "F0", anlegg)
but allows more filters

SETRA-37 Set 1900-01-01 as NULL value for dates

See merge request !47

Add exref (Attestantreferanse)

See merge request !48

Feature/add more endpoints

See merge request !46

Format code with black and add linters

See merge request !42