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MagFieldServices: Rewrite BFieldCond in terms of Eigen3 vectors

Stephen Nicholas Swatman requested to merge sswatman/athena:magfield_eigen into master

The previous code was entirely scalar and quite messy. By rewriting the code in terms of Eigen3 vectors it becomes more expressive and clear. It also becomes a bit faster as we can now leverage SIMD implicitly through Eigen's internal optimisation engine.

We measure a speedup of 14.3% in the getField method. Not huge, but this should drip down into many parts of Athena as a subtle but worthwhile performance increase.

Note: This commit may introduce very slightly different numerical results. This is not a bug, rather it is a consequence of Eigen's internal optimisation heuristics and the way IEEE 754 floating-point mathematics works.

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