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.

Bugfix/optional fields in models.Parameter

See merge request !16

Also a little tidying

FSHO-19: Added update batch method. And fixed format to match the post_new_batch method

See merge request !14

Fsho12, del 1:  mulighet å returnere en liste av batcher uten vouchere

See merge request !13

Add support for api/parameters endpoint

See merge request !12

Fix dateformats

See merge request !10

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

FSHO10: support filtering of batches in list from setra

See merge request !11

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.

Add models and get_batch_errors method

See merge request !9

- 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.

OM46, del 1: Support batch_complete endpoint from setra

See merge request !7

Make Batch.period optional

See merge request !8

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.