Exact match when query for event time

Created by: jkanner

In the latest version of the client, there is an error thrown when trying to get the GPS time of GW190521

from gwosc import datasets
t0 = datasets.event_gps('GW190521')

The error message says that the query for this event name gets multiple events (e.g. includes GW190521_074343). This is not a great behavior, because I really want the GPS time of 'GW190521'. I think when asking for a GPS time, it would be best to demand an exact match in the name (not a substring).

This seems to be a change from version 0.5.8 to 0.6.0.