Generalize broken line method for unstructured grids

[hdrake]

A long-term goal, perhaps as a milestone for sectionate v1.0.0, would be that users can provide sectionate with either a structured xgcm.Grid object or the equivalent unstructured grid object (perhaps using an analagous uxarray class), and sectionate will handle the rest in a completely model-agnostic and generalizable way.

@kailauyeda is working on an adaptation of sectionate's "broken line" algorithm for unstructured grids, starting with a global hexagonal mesh in MPAS-Ocean.