Use same schema in all Magnet.yml payloads

The initial condition format used for Magnet.yml was

---
Measured:
    Current: 5850.0
    Polarity: -1

but the conditions written online use the schema

---
Magnet:
    Current: 5850.0
    Polarity: -1

This MR makes the dummy Magnet condition for run 0 consistent with the rest.