instrument: add get_raw() API

Derived metrics may be calculated form data in raw output that is not
present in the resulting MeasurementCSV. This adds a method to provide
uniform access to raw artifacts generated by an instrument.

devlib/instrument/__init__.py

@@ -297,3 +297,6 @@ class Instrument(object):
 
     def get_data(self, outfile):
         pass
+
+    def get_raw(self):
+        return []