Errors on generating GSP batches because of the national forecast is GSP ID 0

[jacobbieker]

Describe the bug
This comes up in the filtering of the GSP threshold_mw, the set of metadata GSP IDs does not match the set of GSP power IDs because there is an extra GSP ID 0 for national forecasting in the power data, but not the metadata.

To Reproduce
Steps to reproduce the behavior:

1. Go to '...'
2. Click on '....'
3. Scroll down to '....'
4. See error

Expected behavior
A clear and concise description of what you expected to happen.

Additional context
Fix should just be to drop that GSP ID when making batches