implemented variables and function call evaluation

src/main/java/de/hsrm/compiler/Klang/ContextAnalysis.java

@@ -87,6 +87,11 @@ public class ContextAnalysis extends KlangBaseVisitor<Node> {
     return result;
   }
 
+  @Override
+  public Node visitVariable(KlangParser.VariableContext ctx) {
+    return new Variable(ctx.IDENT().getText());
+  }
+
   @Override
   public Node visitAtomExpression(KlangParser.AtomExpressionContext ctx) {
     return this.visit(ctx.atom());