Fix units and long names of derived ERA5 variables

[Blizzeq]

Closes #509.

Changes proposed in this Pull Request

Sets the units and long_name of the derived ERA5 variables that carried attributes from their inputs:

• height: m, Height (was m**2 s**-2, Geopotential)
• wnd_azimuth: rad, 100 metre wind azimuth (was the u100 component's m s**-1)
• solar_altitude / solar_azimuth: long_name solar altitude / solar azimuth (was latitude), set in SolarPosition

Only metadata changes; values are the same. The helpers are shared, so era5-edh gets the same fix. I left the capitalisation of the other long_names (point 4 in the issue) as they are.

test/test_era5_attrs.py builds small synthetic datasets and checks the attributes, so it needs no download; all three tests fail on master. The other offline tests give the same results as on master (the failures there need network or cutouts).

Checklist

• Code changes are sufficiently documented; i.e. new functions contain docstrings and further explanations may be given in doc.
• Unit tests for new features were added (if applicable).
• Newly introduced dependencies are added to environment.yaml, environment_docs.yaml and setup.py (if applicable).
• A note for the release notes doc/release_notes.rst of the upcoming release is included.
• I consent to the release of this PR's code under the MIT license.