DerivedMeasurements: add process_raw() + doc updates

- Add process_raw() method to the API. This is inteneded to be invoked
  on any raw output (i.e. not MeasurmentCsv) generated by an Instrument.
- Both process() process_raw() are portional to be overriden by
  impolementation; the default behavior is to return an empty list.
- The output specification for both is extened to allow
  MeasurementCsv's, as well as DerivedMetric's.
- Documentation has been reworded for clarity.

devlib/derived/__init__.py

@@ -53,6 +53,8 @@ class DerivedMetric(object):
 
 class DerivedMeasurements(object):
 
-    @staticmethod
-    def process(measurements_csv):
-        raise NotImplementedError()
+    def process(self, measurements_csv):
+        return []
+
+    def process_raw(self, *args):
+        return []