Fix parsing -net option (reported by cameron.weadick) and error message with -fast -bb (reported by Remi Denise)

Fix thread-unsafe model_info with merging models step (reported by Marianne and Karen). Start v1.6.1.a

--write-branches option to write to file .branches.csv the branch lengths of partition trees corresponding to super tree (requested by Rob Lanfear)

Fix thread-unsafe model_info with merging models step (reported by Marianne and Karen). Start v1.6.1.a

--write-branches option to write to file .branches.csv the branch lengths of partition trees corresponding to super tree (requested by Rob Lanfear)

correcting mt matrices

refactor partition_type to BRLEN_*. Checkpoint partition_type to prevent mixing edge-linked and edge-unlinked partition finding