Kernel for the vertical derivative of the force on an MFM tip due to mx, my, mz.

Obtains the demag kernel either from cacheDir/ or by calculating (and then storing in cacheDir for next time).

Boltzmann's constant in J/K.

Permeability of vacuum in Tm/A.

Bohr magneton in J/T.

Electron charge in C.

Use 2N-1 padding instead of 2N for sizes up to SMALL_N.