A neater way to add attrs

[pp-mo]

For now, just a placeholder, since some working out + testing required.
WIP
Now functionally complete, and tested.
But it will now be simpler to await #70, to avoid forcing a rebase of that.
ALSO

• add whatsnew entry, somehow ?
• consider documenting in tutorial when it exists Add User Tutorial #73

The objective is to replace e.g. :

NcVariable(
    "var_z", dimensions=("z"), data=np.arange(N_Z),
    attributes=[
        NcAttribute("a", 1),
        NcAttribute("units", "m")
    ]
),

with the simpler ...

NcVariable(
    "var_z", dimensions=("z"), data=np.arange(N_Z),
    attributes={"a": 1, "units": "m"}
),