Adding references for off-diagonal thermal conductivity (#88)

[OrbitalC]

The references are added in the README and the output, and the fact that we use eq. 22-24 from Fiorentino is explained in the doc.
The standard output looks like

                               Software : F. Knoop et al., J. Open Source Softw 9(94), 6150 (2024)
                                 Method : D. A. Broido et al., Appl Phys Lett 91, 231922 (2007)
 Iterative Boltzmann transport equation : M. Omini et al., Phys Rev B 53, 9064 (1996)
                              Algorithm : A. H. Romero et al., Phys Rev B 91, 214310 (2015)
     Off diagonal coherent contribution : M. Simoncelli et al., Nat Phys 15 809-813  (2019)
                                          L. Isaeva et al., Nat Commun 10 3853 (2019)
                                          A. Fiorentino et al., Phys Rev B 107, 054311  (2023)

[flokno]

Thanks @OrbitalC !

[flokno]

Closes #88