Allow single setting to be passed to read_knmi

[dbrakenhoff]

Possibly somewhat related to the list of ToDos in #90.

I'd like to pass a single setting to hpd.read_knmi:

hpd.read_knmi(
    locations=pstore.oseries,
    meteo_vars=("RD",),
    starts="2010",
    ends="2023-01-31",
    settings={"normalize_index": False},
)

Currently, I have to provide all settings before I can get this to work. This can be fixed by updating the default settings with the user-provided settings:

default_settings = _get_default_settings(settings)
if settings is not None:
    default_settings.update(settings)
    settings = default_settings

[dbrakenhoff]

Closed by #101