New _get_time_column method for EventTable
This PR introduces a new internal method EventTable._get_time_column
to try and work out which column (if any) represents 'time' in the current table. This is useful for methods like event_rate()
which require a time column to operate.
I also fixed the examples for table
which were all still referring to the old magic 'time' column from the sngl_burst
table.