ParaxialSurface type implementation for first order layout

[HarrisonKramer]

Description

A paraxial surface is defined solely by its focal length (assumed in air). It can be used at the start of a lens design to build a first order model, after which thicknesses can be added and a complete design can be built and optimized.

Requirements

• Create a new ParaxialSurface type (subclass of Surface)
• ParaxialSurface has a planar geometry
• Implement tracing logic within the class
• Implement paraxial tracing logic
• Add optical path to rays that pass through the (infinitesimally thin) surface by treating the lens as a phase transformer
• Add logic for adding a paraxial surface in the surface factory
• Update visualization of the new surface type
• Add documentation
• Add one example in the learning guide
• Add examples in the gallery
• Add unit tests
• (optional) consider how the surface can easily be converted into a thick lens, perhaps with a "convert_to_thick_lens" method.