Add Field interface

To ensure that all Field classes implement __toString

Author: danmichaelo

src/Fields/FieldInterface.php

@@ -0,0 +1,8 @@
+<?php
+
+namespace Scriptotek\Marc\Fields;
+
+interface FieldInterface
+{
+    public function __toString();
+}

src/Fields/Isbn.php

@@ -2,7 +2,7 @@
 
 namespace Scriptotek\Marc\Fields;
 
-class Isbn extends Field
+class Isbn extends Field implements FieldInterface
 {
     public function __toString()
     {

src/Fields/Subject.php

@@ -2,7 +2,7 @@
 
 namespace Scriptotek\Marc\Fields;
 
-class Subject extends Field
+class Subject extends Field implements FieldInterface
 {
     public static $glue = ' : ';
 

src/Fields/Title.php

@@ -2,7 +2,7 @@
 
 namespace Scriptotek\Marc\Fields;
 
-class Title extends Field
+class Title extends Field implements FieldInterface
 {
 
     /**