Add Lineage/lineage_t concept

[jeromekelleher]

Currently we store the head of the segment chain to represent a lineage in both the Python and C versions. This is getting increasingly unwieldly. Replace with an object representing a lineage, and move the lineage specific things like population and label into this structure.

This will be a noisy change, but a worthwhile one.

FYI I'm on this one @GertjanBisschop

[jeromekelleher]

This is partially done, with #2296 creating the lineage_t struct, and #2297 moving the label, population and hull attributes to this struct in the minimal working way. Follow ups will start to take advantage of this cleaner structure to refactor.