Restore the analytic validation with the corrected sheath sign, measured signed

Card: state the validations that are signed and settled

Pull the annulus validation figure: sign convention unresolved (see dmipy-sim#48)

Card: cross-sections, analytic validation, B0 rotation, and a runnable reproduction snippet

Winther G6 axons v3: 29 replay packs + masked susceptibility field companions

initial commit