VMD Wholespace - B, dBdt

[lheagy]

For both FDEM_MagneticDipole_Wholespace.ipynb, TDEM_MagneticDipole_Wholespace.ipynb, it would be nice to include B (and dBdt for TDEM) for completeness.

@sgkang or @dccowan: would you be willing to take this on?