Allow the external definition of functions in RuleEvaluation (#55)

* Added function to count number of answered questions

* Moved CountDefinedElements from Archie to Chiron

src/main/java/com/nedap/archie/rules/evaluation/RuleEvaluation.java

@@ -44,6 +44,7 @@ public class RuleEvaluation {
     private Archetype archetype;
     private List<Evaluator> evaluators = new ArrayList<>();
     private HashMap<Class, Evaluator> classToEvaluator = new HashMap<>();
+    private FunctionEvaluator functionEvaluator;
 
     //evaluation state
     private Pathable root;
@@ -63,6 +64,7 @@ public class RuleEvaluation {
 
     public RuleEvaluation(Archetype archetype) {
         this.archetype = archetype;
+        this.functionEvaluator = new FunctionEvaluator();
         add(new VariableDeclarationEvaluator());
         add(new ConstantEvaluator());
         add(new AssertionEvaluator());
@@ -71,7 +73,7 @@ public RuleEvaluation(Archetype archetype) {
         add(new VariableReferenceEvaluator());
         add(new ModelReferenceEvaluator());
         add(new ForAllEvaluator());
-        add(new FunctionEvaluator());
+        add(functionEvaluator);
     }
 
     private void add(Evaluator evaluator) {
@@ -110,6 +112,10 @@ public ValueList evaluate(RuleElement rule) {
         throw new UnsupportedOperationException("no evaluator present for rule type " + rule.getClass().getSimpleName());
     }
 
+    public void registerFunction(FunctionImplementation function) {
+        functionEvaluator.registerFunction(function);
+    }
+
     public Pathable getRMRoot() {
         return root;
     }