SOCAT: add overlap/duplicate check

[karlmsmith]

Reported by @karlmsmith on 22 Mar 2017 18:53 UTC
Update the app to detect overlapping and duplicate datasets and run in on v5 to find any. (Camilla found one already - expocode differing by just one day). Need to revise dashboard to report such errors when the user is working with them in the dashboard before they are submitted.

Migrated-From: http://dunkel.pmel.noaa.gov/trac/las/ticket/1743