Move magnet cutting to ` MagnetCoil` class

[gonuke]

The process of cutting a magnet coil might make sense to do with each magnet individually rather than as a set.

This probably warrants some discussion to confirm if this is a wise change, but it further focuses on the operations to be per magnet coil and thus allows a richer set of possible actions.

[connoramoreno]

We might reconsider this issue in the context of the modified magnet modeling workflow. It may still be worth doing, but would look differently in that new context.