[WIP] Recoil generation #288
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Fixes #286
Description
User @dmonah observed deviations in sputtering yields when ions were incident on a flat surface from different directions (e.g., other than in the +x direction).
dmonah did lots of tests that suggested the problem lay in the recoil generation routine. These routines were a big part of the problem, as they introduced two issues for incident directions with non-zero z elements: first, a subtle distortion in the recoil locations is introduced for directions close to +/-z. Second, generated recoils were not consistently located in the plane of atomic deflection. This caused near-surface physics - particularly, low energy reflection and sputtering - to be affected for any incident atoms with a significant +/- z component to their velocity.
To fix this, I re-derived formulas for the recoil location and deflection. The updated formulas keep these directions consistent for all incident directions, fixing the issue
Tests
See comments in #286 for details on the various tests I'm running to make sure this is working correctly.