Use scattering in the model and fix major flaws

[nbud]

• Compute TFM with scattering
• Compute sensitivity images with scattering

• Correct critical bug in the beam-spread computation paths with one or more transmission/reflection:

  Before: (r0 + r1/gamma1 + r2/gamma2)^(-1/2) After: (r0 + r1/gamma1 + r2/(gamma2*gamma1))^(-1/2)

• Correct critical bug for transmission and reflection coefficients: use displacement ratios instead of stress ratios.
• General improvements