Convenience function for new service to get entity's children
Description
The existing query service must be removed. Clients will rely on (1) the file Views and (2) a new service to get the children of an entity, described here:
https://sagebionetworks.jira.com/wiki/display/PLFM/Consistent+List+Entity+Children
This task is to write a convenience function in Python for the new service.
