Feature request/Bugfix: Added support for two part difference scale which was needed for risk-opportunity scale

Body:
- introduced difference scale (and its processing)
- changed risk_opportunity_perception.yaml to new format
- retained single scale for cognitiv_selfesteem and effects_on_work

config/questionnaires/cognitiv_selfesteem.yaml

@@ -27,7 +27,7 @@ scales:
     response_options: "1 = strongly disagree, 7 = strongly agree"
     output: "cognitive_selfesteem_thinking"
     reference: "Ward (2013)"
-
+    retain_single_items: true
 
   - name: "cognitive_selfesteem_memory"
     label: "cognitive_selfesteem memory"
@@ -50,6 +50,7 @@ scales:
     response_options: "1 = strongly disagree, 7 = strongly agree"
     output: "cognitive_selfesteem_memory"
     reference: "Ward (2013)"
+    retain_single_items: true
 
   - name: "cognitive_selfesteem_transactive_memory"
     label: "cognitive selfesteem transactive memory"
@@ -72,3 +73,4 @@ scales:
     response_options: "1 = strongly disagree, 7 = strongly agree"
     output: "cognitive_selfesteem_transactive_memory"
     reference: "Ward (2013)"
+    retain_single_items: true