Benchmark with Mayes et al. (2018)

[ax3l]

Perform the benchmarks described in @ChristopherMayes et al., "3D SPACE CHARGE IN BMAD", IPAC2018 (2018)
DOI:10.18429/JACoW-IPAC2018-THPAK085

and also evaluate performance.

Potentially add as CI test for correctness (note: the paper uses 10^6 particles; for CI purposes we might want to go lower because of constrained resources).

Draft in #257