Path/Line Integrated Diagnostic (Abstract classes + interferometer implementation)

[pheuer]

This PR introduces a new abstract class "PathIntegratedDiagnostic" to the diagnostics module. This class abstracts the source/grid/detector geometry already used in SyntheticProtonRadiography. I've refactored the SyntheticProtonRadiograph module to be a subclass of the new class, but there are no API changes.

This PR also introuduces a subclass of PathIntegratedDiagnostic, LineIntegratedDiagnostic, which represents the case where the integration paths are straight lines. Given a grid object, this class can line-integrate through any quantity on the grid along any arbitrary straight line of sight. The quantity integrated is given by a "_integrand" method, which can be in turn be calculated using any combination of the quantities on the grid. Different diagnostics with non-trivial response functions (like proton radiography) can be implemented by just replacing this _integrand in subclasses.

This module could has many applications (x-ray radiography, linear-regime charged particle radiography, etc.), but for a start I've implemented a very basic interferometry function. Given a 1 mm radius sphere of constant density 3e19 cm^-3, the following is the predicted interferogram with a 263 nm probe beam

The documentation still needs some work (I'll write one notebook explaining the interferometry subclass and another showing how to create and use custom subclasses). But I'm ready for comments on the basic structure of the LineIntegratedDiagnostic class!

[pheuer]

Thanks for the help, documentation wizard @rocco8773 !

[StanczakDominik]

All right, this looks great to me and I could see it being merged - unless @rocco8773 wants to take another sweep, since this falls under diagnostics? (if no, just hit merge!)

Also, is it just me, or does this look like the Batman logo?

[pheuer]

Great! I don't see the batman logo even if I squint really really hard...I think you might need some sleep @StanczakDominik ;)