BEAMS3D: Re-entrant beams

[lazersos]

Tangential neutral beams which aim toward the inboard side of the magnetic axis can pass through a region on the inboard side between the plasma and wall. These particles if not ioninzed on the first pass will reenter the plasma and can ionize again. The current logic for beam deposition will not treat the re-entrance of these particles. They will be treated as shine-through and most likely end up on the outboard side of the wall. Simulations of the ASDEX-U device (@kudav) have highlighted this issue.

This issue proposes a test-problem for the development of a bugfix for this bug. Using a circular cross section tokamak equilibria a wall will be developed which has a rather large minor radius relative to the plasma. Three beamlines will be considered: a radial beam, an inboard aiming tangential beam, and and outboard aiming tangential beam. Additionally a port for each beam will be added to the wall model to verify port loss estimates.

[kudav]

I mentioned a likely fix for this bug in #345 (review)
Essentially, the loop for the multi-pass needs to be extended to the end of beams3d_follow_neut and not exclude the last wall hit check as it did thus far. I think a test-problem based on the existing benchmarks is a good idea, however.