Post new batches without sending None-fields

See merge request !21

Exclude unset values from beeing transfered when posting a new batch to
Setra. Setra does not accept fields set to None (at least period and
vouchertype).

Redefine the interface for post_new_batch()

See merge request !20

This interface avoids leaking transport-specific information, while
returning an indication of how the request fared so the user can decide
on how to proceed with processing.

Clarify responses from post/put

See merge request !18

Before this commit it was unclear whether a post request was successful
or not. We now return the content and status to make this clearer

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

Refactor models and methods for batches

See merge request !17

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.

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.