classifiers: usability updates

- add IterationResult-level classifiers that get merged into every
  added metric (saves having to pass the same classifiers to each
  metric added).
- Added a global alias to csv result processor's  use_all_classifiers
  attribute.

wlauto/result_processors/standard.py

@@ -67,6 +67,7 @@ class CsvReportProcessor(ResultProcessor):
 
     parameters = [
         Parameter('use_all_classifiers', kind=bool, default=False,
+                  global_alias='use_all_classifiers',
                   description="""
                   If set to ``True``, this will add a column for every classifier
                   that features in at least one collected metric.