Add isomer_id function

Author: anu1217

src/nucname.h

@@ -357,6 +357,8 @@ namespace nucname
   int zzaaam_to_id(std::string nuc);
   /// \}
 
+  /// \ function that checks for metastability and returns a number according to the state
+  extern int pyne::nucname::isomer_id(std::string end_char)
 
   /// \name ZZZAAA Form Functions
   /// \{