add inferred return type to IWithKeywordParameters::getParameter to avoid a lot of casting in client code. This should be source-backward compatible, not binary though (#310)

Author: jurgenvinju

src/main/java/io/usethesource/vallang/IWithKeywordParameters.java

@@ -25,7 +25,7 @@ public interface IWithKeywordParameters<T extends IValue> {
      * @param label identifies the parameter
      * @return a value if the parameter has a value on this node or null otherwise
      */
-    public @Nullable IValue getParameter(String label);
+    public <X extends IValue> @Nullable X getParameter(String label);
 
     /**
      * Set the value of an parameter

src/main/java/io/usethesource/vallang/impl/fields/AbstractDefaultWithKeywordParameters.java

@@ -65,9 +65,10 @@ public String toString() {
         return content.toString();
     }
 
+    @SuppressWarnings("unchecked")
     @Override
-    public @Nullable IValue getParameter(String label) {
-        return parameters.get(label);
+    public @Nullable <X extends IValue> X getParameter(String label) {
+        return (X) parameters.get(label);
     }
 
     @Override