Implement DoubleTag

lib/nbt/Tag.dart

@@ -1,5 +1,6 @@
 import 'package:libac_flutter/nbt/impl/ByteArrayTag.dart';
 import 'package:libac_flutter/nbt/impl/CompoundTag.dart';
+import 'package:libac_flutter/nbt/impl/DoubleTag.dart';
 import 'package:libac_flutter/nbt/impl/TagEnd.dart';
 import 'package:libac_flutter/nbt/impl/TagFloat.dart';
 import 'package:libac_flutter/nbt/impl/TagInt.dart';
@@ -88,6 +89,10 @@ abstract class Tag {
         {
           return new CompoundTag();
         }
+      case TagType.Double:
+        {
+          return new DoubleTag();
+        }
       case TagType.End:
         return new TagEnd();
       case TagType.Short:

lib/nbt/impl/DoubleTag.dart

@@ -1,18 +1,27 @@
 import 'package:libac_flutter/nbt/Stream.dart';
 import 'package:libac_flutter/nbt/Tag.dart';
 
-class TagDouble extends Tag {
+class DoubleTag extends Tag {
-  final double value;
+  double _value = 0.0;
-  TagDouble({required this.value});
+
+  DoubleTag();
+
+  DoubleTag._(double value) {
+    _value = value;
+  }
+
+  static DoubleTag valueOf(double value) {
+    return DoubleTag._(value);
+  }
 
   @override
   void readValue(ByteLayer data) {
-    // TODO: implement readValue
+    _value = data.readDouble();
   }
 
   @override
   void writeValue(ByteLayer data) {
-    // TODO: implement writeValue
+    data.writeDouble(_value);
   }
 
   @override