ResourceIdentifier: QuakeML conform by default

 - if no "prefix" is specified, initialize with "smi:local/" prefix to
   make it QuakeML conform
 - avoids possible hazards with ResourceIdentifiers getting changed
   automatically during QuakeML output
 - short test
 - CHANGELOG
clearer implementation and adapt docstring info
Merge pull request #559 from megies/resource_id_prefix

check ResourceIdentifier/public_id handling in event/QuakeML output