EHN: Addition of ensemble variable to ECMWF dictionaries

[LUCKIN13]

Pull request type

• Code changes (bugfix, features)
• Code maintenance (refactoring, formatting, tests)
• ReadMe, Docs and GitHub updates

Checklist

• Tests for the changes have been added (if needed)
• Docs have been reviewed and added / updated
• Lint (black rocketpy/ tests/) has passed locally
• All tests (pytest tests -m slow --runslow) have passed locally
• CHANGELOG.md has been updated (if relevant)

New behavior

With this small change it is possible to use ensemble files from ERA5 database, which might be usefull for Monte Carlo Simulations.

Enter text here...

Breaking change

• Yes
• No

[Gui-FernandesBR]

1. @LUCKIN13 , it is critical that we run slow tests before merging this PR
2. Do you think we could update the documentation to clearly state to the users they can now use this feature?
3. The CHANGELOG needs to be updated.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 80.02%. Comparing base (f17893b) to head (6d10f1a).
⚠️ Report is 18 commits behind head on develop.

Additional details and impacted files

@@            Coverage Diff            @@
##           develop     #842    +/-   ##
=========================================
  Coverage    80.02%   80.02%            
=========================================
  Files           98      103     +5     
  Lines        12004    12527   +523     
=========================================
+ Hits          9606    10025   +419     
- Misses        2398     2502   +104     

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[LUCKIN13]

1. @LUCKIN13 , it is critical that we run slow tests before merging this PR
2. Do you think we could update the documentation to clearly state to the users they can now use this feature?
3. The CHANGELOG needs to be updated.

Will do all that asap

[LUCKIN13]

All slow tests passed.
Funny thing is that this possibility was already in the documentation, even though the ECMWF dictionary didn't have the ensemble parameter.