Add magnetic field controller.

[Relm-Arrowny]

Part of #1875

Adding magnet control and stage.

[fajinyuan]

Could the magnet field cntrol and stage control be split into 2 different issues? or do you means the stage conteol is control of magnet field value?

[Relm-Arrowny]

Could the magnet field cntrol and stage control be split into 2 different issues? or do you means the stage conteol is control of magnet field value?

It could but the stage is 5 lines in the config and I was going to do #1875 as a single pr but it got to big hence this.