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

Instead of two methods we now have one that does the appropriate
action based on the input type.

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

This gives us more options for how to use the function, and makes setra client easier to use in Bookitlab-api (BOOK-17)

batchid- and voucherno_interface are not used.

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