New Feature: Two-magnon scattering

[mducle]

A long-requested feature from users is to be able to calculate the two-magnon continuum from LSWT.

There is two parts to this:

1. A two-magnon density of states
2. The two magnon intensities

Part 1 is a combinatorial problem working out the two magnon energy $\Omega(\mathbf{Q}) = \omega_1(\mathbf{q}_1) + \omega_2(\mathbf{q}_2)$ subject to $\mathbf{Q} = \mathbf{q}_1 + \mathbf{q}_2$ of two single-magnon excitations $\omega_1(\mathbf{q}_1)$ and $\omega_2(\mathbf{q}_2)$ - that is we would need to calculate the kinematically allowed momentum $\mathbf{Q}$ and how many modes could contribute at that point (the density of states).

RW saw a talk at NMSUM where one of Radu's postdoc has implemented this extension to SpinW (Matlab) - we should reach out to them.

Part 2 is more complicated and I'm not sure of the maths yet...

[lucas-wilkins]

I've got the maths for part 2, I actually think that part 1 is the more challenging part from a software perspective.