[feature] Enhance readability/performance with BCA refactor

[drobnyjt]

Is your feature request related to a problem? Please describe.
Currently, the code has a number of near-hacks to perform the binary collision approximation trajectory steps in the correct order. These include storing previous directions/positions in the Particle object, having the electronic stopping calculation broken up for local/nonlocal forms, potentially redundant boundary condition checks, etc.

Proposed solution
Further separating out certain procedures into functions would be helpful. Additionally, an analysis of the BCA order of operations from a programming perspective should be performed prior to the refactor to ensure correctness and simplicity.

[drobnyjt]

Many recent changes have rendered this issue mostly moot - although there is still room for improvement in the BCA core loop, the readability issue has largely been addressed.