Add quad triplet optimization example

[cemitch99]

Added an example of a quadrupole triplet focusing system, illustrating matching of a beam to small spot size at focus (target values of beta_x = beta_y and alpha_x = alpha_y = 0).

This example will be modified to illustrate the use of optimization.

• Add example input with nominal (near-optimal) parameters
• Update examples/CmakeLists.txt
• Update documentation
• Add plot script
• Modify Python script to allow optimization of quad strengths and/or spacing

[n01r]

Thank you, @cemitch99! Looks good 🙂
I marked some tiny details with comments.
Will the last bullet point about the actual optimization also be done in this PR or on a follow-up?

[cemitch99]

Here is an illustration of the beta functions.
Optimized_Triplet.pdf

[ax3l]

Thank you very much!

Let's add the visualization of the beta functions as a visualization script (as we do in the FODO example)?

[cemitch99]

Good idea. Working on the visualization script now.

[ax3l]

Thank you so much. I'll try to add the optimization part this week :)

[ax3l]