Port two-magnon scattering corrections

[kbarros]

We are iterating here to port the two-magnon scattering corrections.

User interface will improve with time. To test, use add Sunny#2magnon and then execute Hao's example below.

[Hao-Phys]

The updated sample code:

using Sunny

a = 1
latvecs = lattice_vectors(a, a, 10a, 90, 90, 90)
cryst = Crystal(latvecs, [[0,0,0]])
latsize = (2, 2, 2)
s = 1/2
J = 1.0
H = 4 * J * s
sys = System(cryst, latsize, [1 => Moment(; s, g=-1)], :dipole; seed=0)
sys = reshape_supercell(sys, [1 -1 0; 1 1 0; 0 0 1])

set_exchange!(sys, J, Bond(1, 1, [1,0,0]))
set_field!(sys, [0.1, 0, H])
randomize_spins!(sys)
minimize_energy!(sys)
energy_per_site(sys)

swt = SpinWaveTheory(sys; measure=ssf_trace(sys), regularization=2e-8)

ωs = 0.0:0.1:5.0
q = 0.61
ret = Sunny.intensities_free_two_magnon(swt, [q, q, 0], ωs, 1e-2; atol=0.5)

using GLMakie
lines(ret)

The expected results