cf-units is a required dependency

[magnusuMET]

cf-units is specified as an optional dependency, but is required: Not having this package installed will give a

NameError: name `Unit` is not defined

when importing pyaro

Error message

>>> import pyaro
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/home/magnusu/Projects/github/metno/pyaro-readers/venv/lib/python3.10/site-packages/pyaro/__init__.py", line 5, in <module>
    from .plugins import list_timeseries_engines, open_timeseries
  File "/home/magnusu/Projects/github/metno/pyaro-readers/venv/lib/python3.10/site-packages/pyaro/plugins.py", line 10, in <module>
    from .timeseries.Engine import Engine as TimeseriesEngine
  File "/home/magnusu/Projects/github/metno/pyaro-readers/venv/lib/python3.10/site-packages/pyaro/timeseries/__init__.py", line 3, in <module>
    from .Reader import Reader
  File "/home/magnusu/Projects/github/metno/pyaro-readers/venv/lib/python3.10/site-packages/pyaro/timeseries/Reader.py", line 4, in <module>
    from .Filter import Filter, filters
  File "/home/magnusu/Projects/github/metno/pyaro-readers/venv/lib/python3.10/site-packages/pyaro/timeseries/Filter.py", line 870, in <module>
    class RelativeAltitudeFilter(StationFilter):
  File "/home/magnusu/Projects/github/metno/pyaro-readers/venv/lib/python3.10/site-packages/pyaro/timeseries/Filter.py", line 877, in RelativeAltitudeFilter
    UNITS_METER = Unit("m")
NameError: name 'Unit' is not defined