Add subclass to modify storage functions

Functions not yet modified
Remove observable table, instead use references to original tables

Remove making of observables table
Add a boolean row for each station to coincidences table
Add s_index to store the station node paths
Store event_index from stations event table in c_index
Store station_id and event_index in c_index
Using station_id as row in s_index the original station node can be
retrieved
Fix storing station_groups in s_index
Remove processing from the search and store function in subclass
Fix CoincidencesESD

Use range instead of arange
Update the description .columns attribute instead of the class
Get the station_group string to store in s_index
Keep `_src_c_index` and `_src_timestamps` in memory

Add search_coincidences to subclass to prevent storing of `_src*` tables
Keeping the data in memory prevents the hdf5 file from growing in size
Modify functions that used the removed tables to use the new class
attributes
Merge branch 'coincidences'

Add subclass of coincidences for ESD
This subclass makes filtering easier
Keep links back to the source events and tables with c_index and s_index