Improving the documentation

[OrtnerMichael]

• Bad example in direct interface !!! uses a loop !!!
• Complex shapes should go to Introduction
• Equations of motion example
• Gradient field example
• Collection local reference frame example (see Rotating collection behaves different than rotating a single magnet #583)
• Improve description of Helmholtz example (see Rotating collection behaves different than rotating a single magnet #583)
• Quadrupol examples shows divide error

[OrtnerMichael]

Equations of motion example
Consider the motion of a magnet in the field of a coil. (simple linear acceleration)

compute force on magnet numerically on a grid
solve equations of motion
make nice plots
This would be a very nice example code based on this request:

• How to model the field distribution of current coil as a Cylinder shape #363

[OrtnerMichael]

Gradient field example

Magpylib fields have a huge advantage over FE fields - they are smooth. This allows computation of gradient fields to a very high level of precision with simple finite differences.

There have been multiple requests for gradient fields in the past - we should make this an example in the gallery.