Apply transverse aperture to thick elements.

[cemitch99]

Adds the comparison of particles against the transverse aperture within thick elements, at the end of each space charge slice. This will close #763 .

Note: Need to verify that using the same name Aperture for the thin aperture element and for the mixin class will not cause conflicts. Otherwise, one of these should be renamed.

• add mixin class
• add application to a drift (as a template)
• address particle id
• add a benchmark example
• add element documentation
• add Python support
• add application to other elements
• add example documentation

[ax3l]

I would move the if into mixin/aperture.H.

[cemitch99]

Note: Windows test fail now, but all tests were previously passing.

[ax3l]

I wonder if for all potential elements, xmax and ymax are general enough argument names.

They do make sense for the Aperture element, but for all other thick elements we might need something more specific, e.g., x_open or x_aperture?

Another small change I thought we could do is use std::optional<amrex::ParticleReal> ... = std::nullopt. That way, we do not redefine 0 with a jump, which technically is a fully blocking aperture. The equivalent in Python is then x_aperture = None, which is clearly no aperture.

What do you think?

[cemitch99]

I like x_aperture, and I'll make that change. Will look into using std::optional.

[ax3l]

To get the value out of std::optional, call operator* or .value().

We will need to write the real value into m_x_aperture because we cannot (should not) use std::optional on GPU. For no value, I would maybe write a negative value into the member variable and only apply the aperture for >=0. That way, 0 is well-defined (blocking).

[cemitch99]

To clarify the last comment: We're avoiding std::optional altogether? If so, I actually prefer the existing approach, to apply the aperture when x_aperture > 0 and y_aperture >0. Why? 1) Allowing for blocking with x_aperture = 0 requires changing the logic/math in the mixin class, 2) A thick element with zero aperture has no use case (eg, its action is the same as a thin screen for blocking), 3) Allowing this case means setting the default values of x_aperture, y_aperture to an arbitrarily-chosen negative number.

[ax3l]

Naming: please see above.