Issue: Optical photon time-of-flight uses incorrect propagation speed in dispersive media
Issue: Optical photon time-of-flight uses incorrect propagation speed in dispersive media: a task in LegoFlow-SWE (Harbor dataset). \ v g = \frac{c}{\,n(E) + E \frac{dn}{dE}\,} \
The task
\[ v_g = \frac{c}{\,n(E) + E \frac{dn}{dE}\,} \]
Part of Lego-X/LegoFlow-SWE.