EMU: Kepco: set limits on hardware appropriately

As an instrument scientist on a muon beamline, I would like the kepco power supply which I use for vertical beam steering to automatically set a large enough voltage limit, so that I can always set a current without needing to worry about whether the voltage limit is high enough.

Notes:
- EMU **always** run this power supply in current mode, never in voltage mode.
- It would be useful to be able to specify the desired mode as a macro so that the kepco never comes up in the "wrong" mode.
- This is **not** required for EMU by cycle (confirmed in discussions with instrument scientists). It is a nice-to-have.
- Instead of using the limits embedded in the kepco hardware, it would be more useful to be able to define explicit voltage and current limits as configuration macros (similar to `genesys` or `danfysik` power supplies)