ENH: Add absolute difference, ,correlation measurements in magfig()

[KCollins]

Description

Model after this slide by James Weygand at AGU24:

Potential impact

This feature would be useful for giving a metric of correlation between conjugate magnetometers.

Potential solution(s)

• Calculate absolute difference
• Plot absolute difference in green (by default)
• Create option to plot Bx, By, Bz for the same magnetometer