Add subclass for Coincidences in ESD

This subclass changes the way coincidences are stored.
It adds a boolean column for each participating station to facilitate filtering.
And adds station_id -> station_group look-up table, to find the source event tables.
The c_index table now stores the event_index from the source table
and the station_id for each event in a coincidence.

Removes the _src_timestamps, _src_c_index and observables tables.