dz/dx, dz/dy
dz/dx = -A/C = Emaj.dz * Ebot.dy - Emaj.dy * Ebot.dz
Emaj.dx * Ebot.dy - Emaj.dy * Ebot.dx
dz/dy = -B/C = Emaj.dx * Ebot.dz - Emaj.dz * Ebot.dx
Emaj.dx * Ebot.dy - Emaj.dy * Ebot.dx
To save computation, 1/C computed only one time, then used
as multiplication factor to compute dz/dx and dz/dy