DOC: add reports about T-matrix Riemann sheets

[Zeyna777]

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See a temporary preview of the reports here:

• TR-026: Riemann sheets of a single-channel $T$ matrix
• TR-027: Riemann sheets of a $T$ matrix for two channels

The report implements and explains the 3D Riemann sheets figures of the PDG by showing exactly which formulas are used. As such the main targets of this report are:

1. Using MatrixSymbol to derive Breit-Wigner with a phase space factor
2. Compare the standard phase space factor to Chew-Mandelstam S-wave
3. Use of the 'new' definition of SignedSqrt as described in Lecture 17 in order to get the correct sub-threshold behavior
4. How to reach the second sheet from the standard S-wave Chew-Mandelstam

In https://github.com/ComPWA/K-matrix-research/issues/49, this will be extended into two channels, where we need to chose between Riemann sheets in the region between the two thresholds. But in itself these points are already worthy of its own, comprised report to document the sub-threshold behavior for the two Riemann sheets before moving on to the four Riemann sheets in two channels.

Tasks

Beta Give feedback

1. Remove sliders and change to plotly
2. Plotly plots of Riemann sheets are not showing on ComPWA webpage #206
   2 of 2
   🐛 Bug 📋 Task +2
   
3. Import square root definition (PosArg and SignedSqrt) from Lecture 17 for phase between -π and +π
4. Derive Breit-Wigner from single-channel K-matrix
5. Symbolic expressions for standard phase space factor and Chew-Mandelstam
6. Compare the two phase space factors in 1D plot (real part should be same above threshold)
7. Link to Fig 50.4 to compare PDG CM behavior
8. Find symbolic expression for second CM Riemann sheet)
9. assert that difference above threshold is 2iρ
10. Visualize both CM Riemann sheets in 3D with plotly
11. Do the plots with pcolormesh
12. (?) New visualizations from the PDG (omega and k)
13. Explain how to associate the sheets to physical and unphysical
14. Split T-matrix visualization into phys., unphys., and transition plot (see Fig 50.1)
15. Explanation of what we see and why this implementation works (add links to references)
16. Finalize: renumber TR notebook

Tasks from https://github.com/ComPWA/RUB-EP1-AG/issues/102

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1. Move extra notebooks under temporary-kmatrix/ to K-matrix-research repo
2. TR-026: Make yellow line for branch cut start at branch point (3D plot)
3. TR-027: Indicate thresholds on 2D plots in 3D plot
4. TR-027: Zoom in to 3D plots
5. TR-027: Improve 3D labels (put TI before unphysical)
6. TR-027: Add titles to subplots
7. TR-027: Explain singularities in the plot (which are relevant, which not)
8. TR-027: Some explanation of final 3D plot
9. TR-027: Send PDF for feedback
10. Switch to g0 instead of gamma0
11. Reopen this PR as @Zeyna777

Closes https://github.com/ComPWA/K-matrix-research/issues/46
Closes https://github.com/ComPWA/K-matrix-research/issues/49

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[redeboer]

Perfect, LGTM!