Resolve "FourVector eta is missing protection for a particle along the beam/z axis"
requested to merge 81-fourvector-eta-is-missing-protection-for-a-particle-along-the-beam-z-axis into master
Closes #81 (closed)
Not usually critical, but ATLAS is running with FPE tracing, meaning that "uncontrolled" div-by-zeros are reported as errors (rather than just expected numerics). So it makes sense to anticipate this overflow in the eta (and to a lesser extent rapidity) methods.