add dv::value.getType(), .empty()

src/data/dv.hpp

@@ -292,11 +292,18 @@ namespace dv {
 
         const objects::Object& asObject() const;
 
+        value_type getType() const {
+            return type;
+        }
+
         const size_t size() const;
 
         const size_t length() const {
             return size();
         }
+        bool empty() const {
+            return size() == 0;
+        }
     };
 
     using reference = value&;