Added relationship neuron PARTOF nervous system per user (Vicky Yao) suggestion.
Added new and reworked sex myoblast, SM lineage terms.
Corrected VR muscle numbering, per Zeynep Altun e-mail.
routine update.
Fixed reversed identity between R5 and R6 wrt lineage branch, prompted by
Zeyep Altun.
Manually reconciled secondary IDs of R5 and R6 cells, problems caused by
merges of terms incorrectly earlier. Can't un-merge within OBO-EDIT.
Merge remote-tracking branch 'upstream/master'
adding xrefs to RO terms for nucleus-nucleus relationships