Add <string> fragment

Removes <string> include specifically for clang
Cuts down on duplicate #include <string> in general

Lib/typemaps/fragments.swg

@@ -149,8 +149,12 @@
 #endif
 %}
 
-%fragment("<stddef.h>", "header")  %{
-  #include <stddef.h>
+%fragment("<stddef.h>", "header") %{
+#include <stddef.h>
+%}
+
+%fragment("<string>", "header") %{
+#include <string>
 %}
 
 %fragment("SWIG_isfinite","header",fragment="<math.h>,<float.h>") %{

Lib/typemaps/std_string.swg

@@ -8,10 +8,8 @@
 
 %include <typemaps/std_strings.swg>
 
-%{
-#include <string>
-%}
-  
+%fragment("<string>");
+
 namespace std
 {
   %naturalvar string;

Lib/typemaps/std_wstring.swg

@@ -7,8 +7,8 @@
 
 %{
 #include <cwchar>
-#include <string>
 %}
+%fragment("<string>");
 
 namespace std
 {

Lib/typemaps/traits.swg

@@ -21,18 +21,12 @@
 // Common code for supporting the STD C++ namespace
 //
 
+%fragment("<string>");
 %{
-#include <string>
 #include <stdexcept>
 %}
 
-%fragment("StdStringInclude","header") %{
-#ifdef __clang__
-#include <string>
-#endif
-%}
-
-%fragment("Traits","header",fragment="StdStringInclude") 
+%fragment("Traits","header",fragment="<string>")
 {
 namespace swig {  
   /*