Utils - combine

- Implemented combine, which takes two integer vectors and combines
  them resulting in a single vector contianing the unique values of
  both.

src/util.h

@@ -72,6 +72,7 @@ int execute (const std::string&, std::vector<std::string>);
 #endif
 
 std::string compressIds (const std::vector <int>&);
+void combine (std::vector <int>&, const std::vector <int>&);
 
 #endif
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