OOMMF extension: spin-transfer torque with arbitrary current direction (Zhang Li)

Generalisation of the Zhang Li spin-transfer torque extension (Anv_SpinTEvolve) of OOMMF (https://math.nist.gov/oommf/), initially written by Antoine Vanhaverbeke (https://www.zurich.ibm.com/st/nanomagnetism/spintevolve.html). The original version of the extension can be seen in this commit, it is distributed as part of OOMMF and taken from the OOMMF source distribution (further details are available under https://math.nist.gov/oommf/contrib/oxsext/). The modified version supports arbitrary current directions (3d).