Add pad_string function to string_mod

Author: dongli

src/string_mod.F90

@@ -5,6 +5,7 @@ module string_mod
   private
 
   public to_string
+  public pad_string
 
   interface to_string
     module procedure integer1_to_string
@@ -119,4 +120,14 @@ function logical_to_string(x) result(res)
 
   end function logical_to_string
 
+  function pad_string(x, w) result(res)
+
+    character(*), intent(in) :: x
+    integer, intent(in) :: w
+    character(w) res
+
+    res = x
+
+  end function pad_string
+
 end module string_mod