Compute length of a link automatically, given complex geometry

[KasiaKoz]

Consider the complex geometry of the link, if available, when computing length for the link.

TODOs:
https://github.com/arup-group/genet/blob/main/genet/core.py#L836
https://github.com/arup-group/genet/blob/main/genet/core.py#L878