Add function for accessing ERA5

[kandersolar]

• Closes ERA5 single-point time series database #2420
• I am familiar with the contributing guidelines
• Tests added
• Updates entries in docs/sphinx/source/reference for API changes.
• Adds description and name entries in the appropriate "what's new" file in docs/sphinx/source/whatsnew for all changes. Includes link to the GitHub Issue with :issue:`num` or this Pull Request with :pull:`num`. Includes contributor name and/or GitHub username (link with :ghuser:`user`).
• New code is fully documented. Includes numpydoc compliant docstrings, examples, and comments where necessary.
• Pull request is nearly complete and ready for detailed review.
• Maintainer: Appropriate GitHub Labels (including remote-data) and Milestone are assigned to the Pull Request and linked Issue.

As described in #2420, the ECMWF CDS now provides a simple and fast API for retrieving timeseries ERA5 data. No need for any new dependencies (not even cdsapi), although the nature of the API requires a sequence of requests, and one poll-and-wait step, to complete.

[AdamRJensen]

Discussion on available parameter though the ERA5 time series API can be found here.

[AdamRJensen]

Right now you can't pass pvlib variable names as strings:

This doesn't work:

variables = 'ghi' data, meta = pvlib.iotools.get_era5( latitude, longitude, start, end, variables, api_key, timeout=60)

[AdamRJensen]

This is a phenomenal contribution 🥳

I do suggest renaming the module to "era5" instead of "ecmwf" as I foresee we might add other datasets such as the IFS forecasts, where it would be preferred to have the code in different files.

[AdamRJensen]

Could consider reducing this to 0.5