This commit makes the interface of put_update_batch() quit similar to
post_new_batch().

The Setra API does not accept null values for attributes, but it accepts
that the attribute is omitted. We should probably accept null values
for fields in order to avoid obscure bugs like this, but it does not
hurt to omit the values.

At least we use less of the bandwidth! :)

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).

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.

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.

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

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.