n-body benchmark improvements

[bernhardmgruber]

• add a manually vectorized SoA variant
• allow to disable update() to benchmark move() on large particle counts
• plot move() together with update() on second ordinate
• centralizing choice of Vc vector type (allows switching to SIMD arrays in one place)
• minor changes to text and plot outputs (printing SIMD lanes, using kibi instead of kilo particles)