Distinguish branch 'length' from 'count'.

doc/src/example_config.toml

doc/src/learn.md

doc/src/likelihood.md

doc/src/output.md

doc/src/preprocess.md

doc/src/use.md

src/bin/aphid/main.rs

src/config.rs

src/config/expand.rs

src/config/raw.rs

src/gene_tree.rs

src/gene_tree/triplet.rs

src/learn.rs

src/model/likelihood.rs

src/output/config.rs

src/output/csv.rs

src/output/detail.rs