add variables to forecast_generic

[peterdudfield]

Pull Request

Description

• add variables to forecast_generic for pvnet. Add NWP_UKV_ZARR_PATH and NWP_ECMWF_ZARR_PATH

This is needed with https://github.com/openclimatefix/pvnet_app/pull/48

How Has This Been Tested?

Please describe the tests that you ran to verify your changes. Provide instructions so we can reproduce. Please also list any relevant details for your test configuration

• [ ] 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