Update Subfield.php

Allow subfield code  '#'.

src/HAB/Pica/Record/Subfield.php

@@ -38,7 +38,7 @@ class Subfield
      *
      * @var string
      */
-    public static $validSubfieldCodePattern = '/^[a-z0-9]$/Di';
+    public static $validSubfieldCodePattern = '/^[a-z0-9#]$/Di';
 
     /**
      * Return true if argument is a valid subfield code.
@@ -149,4 +149,4 @@ class Subfield
     {
         return $this->getValue();
     }
-}
\ No newline at end of file
+}