bug fix: current conversion

[samjrholt]

Bug in the conversion from u to j.

The equation previously being used was:

$$ \mathbf{u} = \mathbf{J} \frac{Pg\mu_\text{B}}{2eM_s}$$

based on https://www.zurich.ibm.com/st/nanomagnetism/spintevolve.html . We have now updated this to:

$$ \mathbf{u} = \mathbf{J} \frac{Pg\mu_\text{B}}{2eM_s} \frac{1}{1+\beta^2}$$

based on J. Appl. Phys. 128, 243902 (2020); https://doi.org/10.1063/5.0024382

[samjrholt]

@swapneelap would you mind taking a look through this because me need it merged prior to to submitting the MMM paper? Hopefully it should be pretty quick