Allow multi-magnet systems with the main `magnetforces` approach

[wspr]

Currently magnetforces takes a single fixed magnet and a single floating magnet, which are defined using object-like structures.

There's no reason that magnet_fixed couldn't be an array of structures, so allow for multiple magnets. This would be a slightly different use case than the multipoleforces code but the latter could benefit from a rework to the former.

To make this work, magnets need to be able locate themselves with a relative offset from a local coordinate system.