The ordenr_ubw attribute is never sent in, only returned by the Setra
API. The model handles both input and output of salesorders, so we'll
have to mark the attribute as optional and 

The order number is transferred along with the status of import back to
BookitLab.

it will not fill these field, because they are not yet available, and therefore it would fail when parsing the data 

Setra supports vouchertype being optional, so the client should as well.

This allows more readable code, as we can do

  if batch.batch_progress == BatchProgressEnum.VALIDATION_FAILED:

instead of

  if batch.batch_progress == str(BatchProgressEnum.VALIDATION_FAILED):

or

  if batch.batch_progress == BatchProgressEnum.VALIDATION_FAILED.value:

Validation rule 12 defines period as an optional field. If empty, it
will be populated by master data.

Instead of mixing the Batch model for batches sent *to* with batches
received *from* SETRA, we switch to using an InputBatch and an
OutputBatch. The InputBatch contains the fields that can be used when
sending a Batch (with Vouchers and Transactions), and the OutputBatch
contains all the information one receives from SETRA when fetching a
single batch from the batch/ endpoint, or multiple using the search
functionality.

The get_batch method is reduced to only fetch a single batch, while the
new search_batches method takes the role of searching based on creation
date and interface. In addition, the return_list_of_obj parameter is
removed in favor of the return_objects class attribute.

This commit expects that SETRA includes the id field in Batches.

Also a little tidying

SETRA has a default value for this field, making it optional.

Setra expects datetime.datetime, not datetime.date
In addition, voucherdate is in fact not optional.

- Both get_batch_errors and get_batch_complete now return objects
  unless the client parameter return_objects is set to False
- Tests for models, endpoints and client methods are included.

Validation rule 12 on Batches in SETRA sets the default value before
transfer to Unit4ERP(UBW) if it is not set. The attribute is thus not
required.

There are validation rules in SETRA that sets this value to a default
value set in the parameters table of SETRA. For this to be possible
we must allow users to leave out the value.

Also updated and fixed tests, fixtures and readme

And added two new tests

batchid- and voucherno_interface are not used.

They where originally added because someone thought we should enforce
relations in software 

Introduces SetraClient. The client is made
for use against the SETRA(SEntralt TRAnsaksjonslager) API.

There are models for Batch, Voucher, Transaction and Multi, where Multi is
a combination of the preceding, The Multi model validates that the batchid
of the voucher matches the batch included, and that the voucherid of the
transactions matches one of the included vouchers.

Tests have also been included for the client itself, the endpoints, and
the models.

Configuration for tox, Jenkins and gitlab-ci are included for running
tests and building the client.