[sara-mokhtari]

Description

Fixes #772
Adds a pure-Python bifrustum model (truncated bipyramid with a regular polygonal cross-section).
Depends on #781 (support for pure Python 2D models): this PR is based on py-Iqabc and targets it,
so the diff only shows the bifrustum. Once #781 is merged, I will retarget this PR to master
and merge master into this branch.

• Form factor computed by decomposing the shape into tetrahedra (Delaunay), with the singular cases
  handled by the closed-form expressions of Yang et al. (doi:10.1107/S160057672201130X)
• 1D: orientation average with a Fibonacci quadrature (500 points)
• 2D: Fqabc/Iqabc with theta, phi, psi

Note for review: computational optimizations were added (shared vertices: explained inside the code and documentation).

How Has This Been Tested?

• model_test passes
• F(q) checked against an independent method (Debye equation with Debye calculator: https://github.com/FrederikLizakJohansen/DebyeCalculator) for different truncation ratios. The Debye equation is based on atomic positions while SasView model is based on analytical expressions.

• Tested on experimental data (gold truncated bifrustum: pentagonal cross-section, truncation ratio of ~ 0.2) were fitted as well for different sizes: fit values are similar to TEM values (figure below is an example of fit, the model was also tested on 3 other experimental data).

Review Checklist:

[if using the editor, use [x] in place of [ ] to check a box]

Documentation (check at least one)

• There is nothing that needs documenting
• Documentation changes are in this PR
• There is an issue open for the documentation (link?)

Installers

• There is a chance this will affect the installers, if so
  • Windows installer (GH artifact) has been tested (installed and worked)
  • MacOSX installer (GH artifact) has been tested (installed and worked)
  • Wheels installer (GH artifact) has been tested (installed and worked)

Licensing (untick if necessary)

• The introduced changes comply with SasView license (BSD 3-Clause)

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[pkienzle]

Code looks okay. Models run.