Add graph-based linejoin

[ScottWales]

Version of linejoin using Scipy's graph tools to find paths.

Creates a graph of the input points weighted by distance, then find minimum spanning subtrees of that graph

[ScottWales]

Comparison of linejoin_graph (black dots) vs. linejoin (red lines). Smooth fronts are pretty much the same, there are differences in squiggly fronts (possibly these are marginal detections?)

linejoin_graph completed in ~ 200 ms, linejoin in 1 min 25 s for a 500x500 subset of ERA5.

[ScottWales]

Possible enhancements

• Measure distances in x-y-z space instead of lat-lon
• Return output as a geopandas table