framework/output: save classifiers in Result

Save classifiers at Result as well as Metric level. Reason: when
processing output, one might want to filter complete results, as well as
individual metrics. While it is in theory possible to get the
classifiers for a job by simply extracting the common classifiers
between all metrics, this fails when there are no metrics generated for
a job (note that one might still want to process the output in this
case, e.g. for the artifacts).

wa/framework/job.py

@@ -1,4 +1,5 @@
 import logging
+from copy import copy
 from datetime import datetime
 
 from wa.framework import pluginloader, signal, instruments
@@ -56,6 +57,10 @@ class Job(object):
         else:
             self.workload = self._workload_cache[self.id]
 
+    def set_output(self, output):
+        output.classifiers = copy(self.classifiers)
+        self.output = output
+
     def initialize(self, context):
         self.logger.info('Initializing job {} [{}]'.format(self.id, self.iteration))
         with signal.wrap('WORKLOAD_INITIALIZED', self, context):