Foreseen improvements and fixes

[rizac]

[. ] Drop isf format for isc (see #41 ) Also decide the format parameter in events: is it added or not? in download.yaml we state one thing, in the code another one
[. ] add quakexml as well as stationxml to events / think about better naming
[. ] Refactor database: station identified by net-sta code. Implement migration scripts
[. ] auto adapt max_concurrent_downloads according to responses (*)
[.] simpler download config

(*) Refs:

From:

• iris
• http:[slash][slash]service.scedc.caltech.edu/fdsnws/dataselect/1/

2023-08-08 max_concurrent_downloads: null (auto)

Number of segments per data center url (row) and response type (column):
                                                 Segment  Internal         
                                 No       Url    Not      Server           
                           OK    Content  Error  Found    Error     TOTAL  
-------------------------  ----  -------  -----  -------  --------  -------
service.scedc.caltech.edu  2177   116244   1032    29853   5135721  5285027
service.iris.edu              3      144      0        0         0      147
TOTAL                      2180   116388   1032    29853   5135721  5285174

COLUMNS DETAILS:
 - OK: Data saved (download completed, no additional warning)
 - No Content: No data saved (download completed, the server returned 0 bytes of data)
 - Url Error: No data saved (download failed, generic url error: timeout, no internet connection, ...)
 - Segment Not Found: No data saved (download completed, segment data not found, e.g., in a multi-segment request)
 - Internal Server Error: No data saved (download failed: Server error, server response code 500)

2023-08-09 max_concurrent_downloads: 2

Number of segments per data center url (row) and response type (column):
                                                    Segment  Internal         
                                   No       Url     Not      Server           
                           OK      Content  Error   Found    Error     TOTAL  
-------------------------  ------  -------  ------  -------  --------  -------
service.iris.edu                0        3       0        0         0        3
service.scedc.caltech.edu  105840  3333588  388546  1332341      7262  5167577
TOTAL                      105840  3333591  388546  1332341      7262  5167580

COLUMNS DETAILS:
 - OK: Data saved (download completed, no additional warning)
 - No Content: No data saved (download completed, the server returned 0 bytes of data)
 - Url Error: No data saved (download failed, generic url error: timeout, no internet connection, ...)
 - Segment Not Found: No data saved (download completed, segment data not found, e.g., in a multi-segment request)
 - Internal Server Error: No data saved (download failed: Server error, server response code 500)