Add observation methods

[rubencalje]

Modflow 6 supports adding observation for model outputs, which can make the evaluation of the model easier. Right now there are no methods in nlmod to support observations, but one can use the flopy methods for this. However, some utilities would be welcome in nlmod, for example:

• add observations for rivers, drains, wells or other boundary packages (possibly together with boundname)
• generate observations for the downward flow (formerly known as flow lower face) in an entire model layer, and generate a DataArray for the entire model grid again.

[rubencalje]

For the flow lower face: we can also use this information from the FLOW-JA-FACE budgets in the budget file, when save_flows=True in the npf-package. Therefor, I added the method nlmod.gwf.output.get_flow_lower_face in PR #327, that works for both structured and vertex grids.