Draft: Add prototype for internal output validation

The ReductionWriter creates an internal state about the output based on the reduction operations emitted, which the core can check against the actually written files. This is a draft how this could look like, implementing it only for remove-sources and remove-keys for now.

Must be turned via passing --validate.

@danilo Is this what you had in mind from a structural perspective?

src/exdf/cli/reduce.py

@@ -50,6 +50,10 @@ def _parse_args(argv):
              'excluded from the input data, by default all trains are '
              'considered ')
 
+    ap.add_argument(
+        '--validate', action='store_true',
+        help='validate data found in output files with internal state')
+
     for ep in iter_entry_points('exdf.data_reduction.method'):
         ep.load().arguments(ap)
 
@@ -261,6 +265,9 @@ def main(argv=None):
         log.info(f'Writing voview to {output_path.parent}')
         writer.write_voview(output_path)
 
+    if args.validate:
+        writer.validate()
+
 
 if __name__ == '__main__':
     main()