Update PVNet datapipe to support ECMWF

[jacobbieker]

Pull Request

Description

This adds support for loading different providers of NWP data in the common datapipe utility, focused primarily on PVNet forecasting.

Fixes #

How Has This Been Tested?

Unit tests

• Yes

Checklist:

• My code follows OCF's coding style guidelines
• I have performed a self-review of my own code
• I have made corresponding changes to the documentation
• I have added tests that prove my fix is effective or that my feature works
• I have checked my code and corrected any misspellings

[codecov]

Codecov Report

Merging #229 (39d7bbb) into main (2c39650) will increase coverage by 0.02%.
Report is 2 commits behind head on main.
The diff coverage is 77.77%.

❗ Current head 39d7bbb differs from pull request most recent head d17bdc7. Consider uploading reports for the commit d17bdc7 to get more accurate results

@@            Coverage Diff             @@
##             main     #229      +/-   ##
==========================================
+ Coverage   80.11%   80.13%   +0.02%     
==========================================
  Files         127      127              
  Lines        5611     5614       +3     
==========================================
+ Hits         4495     4499       +4     
+ Misses       1116     1115       -1     

Files	Coverage Δ
ocf_datapipes/training/common.py	98.08% <ø> (ø)
ocf_datapipes/training/example/nwp_pv.py	89.28% <100.00%> (ø)
ocf_datapipes/load/nwp/nwp.py	92.85% <71.42%> (-4.58%)	⬇️

... and 1 file with indirect coverage changes

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[dfulu]

Could make this slightly tidier using:
if configuration.input_data.nwp.nwp_provider in [ "UKMetOffice", "ukv"]?