More tests and documentation on class attributes in IngestReader to configure prescribed values

[RKrahl]

I merged #161 a little bit prematurely, a few more things should have been included:

• add tests to verify that the values of the new class attributes Dataset_complete and DatasetType_name are indeed properly set in the ingested data,
• add some more documentation on that new feature, including some caveats,
• while we are at it, extend the existing test for the _environment element to also add a custom attribute.