SASView Form Factor models for octahedron shape including truncation

Models are computed from analytical expression of the Form factor.

• Octa_general_Lebedev.ipynb contains usage exemple of the model
• octa-general.py is a pure python model where orientation average is computed using Lebedev quadrature
• Lebedev quadrature is implemented using the open source library: https://pypi.org/project/pylebedev/
• folder ./models_with_c_code/ contains other SASView models with ancillary c code where orientation average is computed using Gauss-Legendre quadrature.