feat: gradient distance metric

[william-silversmith]

Experimenting with a hill climbing distance metric as requested in #26

Thanks for the inspiration in https://github.com/ReubenDo/dijkstra3d/blob/master/dijkstra3d.hpp

dijkstra3d.dijkstra(
  data, source, target, 
  metric="gradient", 
  metric_args=[1.0, 1.0], # equal weight to euclidean distance and field intensity
  anisotropy=(1,1,1) 
)