ERA5 fix to allow pressure-levels data with one pressure level

[jsmretschnig]

Behavior so far:

• downloading ERA5 pressure-level data for a single pressure level caused an issue, as "level" was assigned to the dataset twice (due to the CDS-Beta compatibility bugfix)
• downloading ERA5 pressure-level data works fine for multiple pressure levels
• downloading ERA5 single-level data works fine

Now:

• downloading ERA5 pressure-level data also works for a single pressure level

Tests

• QC test locally (make test)
  -- 6 failed, 1017 passed, 161 skipped, 1 xpassed
  -- all the failed tests are GOES tests, and I manually skipped LEO tests
• CI tests pass

Reviewer

Thanks @zebengberg !

[zebengberg]

Thanks for fixing this! I made one suggestion that doesn't change the runtime behavior.