Add spin-torque transfer terms (need field-like and torque-like)

[mangerij]

Need to generate spin waves with spin-torque transfer observing both field-like and spin torque-like components:

m x jt

m x jt x m

See Garello et al Nat. Nanotech. 8, pages 587-593, (2013). Note that jt is the transverse direction the spin current in the SOT layer.

Additional Kernels.

[mangerij]

Closed - too much confusion in the literature especially for antiferromagnets