3-axis electromagnetic coil probe

This is a supporting frame for three perpendicular electromagnetic coils as a handheld probe. This device may be useful for

* detecting/measuring electromagnetic interference (EMI/RFI)
* electroacoustic music

The different orientations of the coils mean that a field of any polarization will be picked up by at least one of the coils. (Note that this is no longer the case if you then sum/mix the output signals because there can then be phase cancellations.) For sound-related purposes, they also allow for collecting multiple timbres and stereo effects.

Quick demo: [picking up noise from a cordless screwdriver.](https://www.youtube.com/watch?v=7gR-ICs26C8)

Also on GitHub: https://github.com/kpreid/3dp-3-axis-coil