Easy way to switch to ECMWF only model

[peterdudfield]

Detailed Description

It would be great to have a really easy way to switch to ECMWF only model

Context

• satellite and ukv data sources sometimes goes down, ecmwf is reliable
• Currernyl need to set a fwe different ENV VARs to make ECMWF only model run

Possible Implementation

• Add a flag that only runs the ecmwf model
• Add task in airflow, if pvnet_app fail, runs ecmwf only model
• Add logic in blend, to use ecmwf forcast after x hours

[peterdudfield]

We might need to openclimatefix/PVNet-summation#26 before deploying this solution

[peterdudfield]

Deployed latest 2.4.13 on dev, and using old models, to check they work.
This can then be deployed to production after a day or so