OEC-358, add secondary section cross-listings query

'rake oec:courses' output was missing secondary cross-listings external to the OEC dept set. This commit adds 'get_secondary_cross_listings' query and reusable SQL fragment.
OK, I think I finally worked this out, but dim bulbs like me could benefit from a descriptive comment here, @johncrossman . Is "This SQL query defines informally cross-listed sections as secondary sections which share a meeting time and place and which include enrolled students." about right?