added clarity and example of obj_id usage
added clarity and example of obj_id usage

opotowsky/cyclus.github.com,gidden/cyclus.github.com,opotowsky/cyclus.github.com,rwcarlsen/cyclus.github.com,gidden/cyclus.github.com,rwcarlsen/cyclus.github.com,rwcarlsen/cyclus.github.com,opotowsky/cyclus.github.com,gidden/cyclus.github.com