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.