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

config/questionnaires/effects_on_work.yaml

@@ -21,3 +21,4 @@ scales:
     response_options: "1 = strongly disagree, 5 = strongly agree"
     output: "effects_on_work"
     reference: "self"
+    retain_single_items: true

config/questionnaires/risk_opportunity_perception.yaml

@@ -7,30 +7,42 @@ scales:
       - id: "rop_1"
         text: "How concerned, if at all, are you when you think about AI?"
         inverse: false
+        part: "risk"
       - id: "rop_2"
         text: "How pessimistic, if at all, are you when you imagine the future use of AI?"
         inverse: false
+        part: "risk"
       - id: "rop_3"
         text: "How likely would AI negatively influence your life?"
         inverse: false
+        part: "risk"
       - id: "rop_4"
         text: "How likely would you suffer from the implementation of AI into everyday life?"
         inverse: false
+        part: "risk"
       - id: "rop_5"
-        text: " How confident, if at all, do you feel when you think about the potential of AI?"
+        text: "How confident, if at all, do you feel when you think about the potential of AI?"
         inverse: false
+        part: "opportunity"
       - id: "rop_6"
         text: "How optimistic, if at all, are you when you imagine the future use of AI?"
         inverse: false
+        part: "opportunity"
       - id: "rop_7"
         text: "How likely would AI positively influence your life?"
         inverse: false
+        part: "opportunity"
       - id: "rop_8"
         text: "How likely would you benefit from the implementation of AI into everyday life?"
         inverse: false
+        part: "opportunity"
     score_range: [1, 5]
-    format: "bipolar"
-    calculation: "mean"
+    format: "Lickert"
+    calculation: "difference"
+    parts:
+      minuend: "opportunity"    # score = mean(minuend) - mean(subtrahend)
+      subtrahend: "risk"
+    retain_subscales: true       # optional: also emit the two pole means
     response_options: "1 = not at all, 5 = extremely"
-    output: "risk_opportunity_score"
+    output: "risk_opportunity_perception"
     reference: "Adapted from Walpole & Wilson, 2021; Schwesig et al., 2023"