[feature] Major refactor to enhance code organization, potentially improve performance

[drobnyjt]

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Is your feature request related to a problem? Please describe. RustBCA has some code consistency issues and organizational problems that should be fixed with a major refactor at some point in the future. Below is a non-complete list of the issues I'm aware of:

• interactions should be re-implemented as traits - see Issue #132
• structs should be made private, with fields accessible by method where needed
• some functions should be made methods; in particular:

1. choose_collision_partner in bca.rs
2. surface_refraction and refraction_angle in particle.rs
3. particle_advance and rotate_particle in particle.rs
4. boundary_condition_planar and surface_binding_energy in material.rs

• SoA (Struct of Arrays) should be considered for Particles over AoS - could improve performance significantly
• Electronic stopping should be broken into component functions so bits and pieces can be unit-tested

Additional context Biggest effect of this refactor will be easing development of alternative ways of running the code, preparing the code to be made a library for binary collisions first with the physics loop and I/O being optional, and making more parts of the code unit-testable.