[#159] Remove non-static initializers

Remove non-static initializers except one (keep one as an example)

src/main/java/seedu/address/model/AddressBook.java

@@ -18,6 +18,13 @@ public class AddressBook implements ReadOnlyAddressBook {
     private final UniquePersonList persons;
     private final UniqueTagList tags;
 
+    /*
+     * The 'unusual' code block below is an non-static initialization block, sometimes used to avoid duplication
+     * between constructors. See https://docs.oracle.com/javase/tutorial/java/javaOO/initial.html
+     *
+     * Note that non-static init blocks are not recommended to use. There are other ways to avoid duplication
+     *   among constructors.
+     */
     {
         persons = new UniquePersonList();
         tags = new UniqueTagList();

src/main/java/seedu/address/storage/XmlSerializableAddressBook.java

@@ -25,20 +25,19 @@ public class XmlSerializableAddressBook implements ReadOnlyAddressBook {
     @XmlElement
     private List<XmlAdaptedTag> tags;
 
-    {
-        persons = new ArrayList<>();
-        tags = new ArrayList<>();
-    }
-
     /**
      * Empty constructor required for marshalling
      */
-    public XmlSerializableAddressBook() {}
+    public XmlSerializableAddressBook() {
+        persons = new ArrayList<>();
+        tags = new ArrayList<>();
+    }
 
     /**
      * Conversion
      */
     public XmlSerializableAddressBook(ReadOnlyAddressBook src) {
+        this();
         persons.addAll(src.getPersonList().stream().map(XmlAdaptedPerson::new).collect(Collectors.toList()));
         tags.addAll(src.getTagList().stream().map(XmlAdaptedTag::new).collect(Collectors.toList()));
     }