text: #include<string>
  
  /*
  Write prototype for function get_gc_content that accepts
  a const reference string parameter and returns a double
  */
  double get_gc_content(const std::string& dna);
  
  /*
  Write prototype for function get_reverse_string that 
  accepts a string parameter and returns a string
  */
  std::string reverse_string(std::string str);
  
  /*
  Write prototype for function get_dna_complement that
  accepts a string and returns a string
  */
  std::string get_dna_complement(std::string dna);
  