Adds hydrogens

        Args:
            num (int): number of hydrogens

def add_hydrogen(self, num):
        """Adds hydrogens

        Args:
            num (int): number of hydrogens
        """
        self.H_count = num
        if num > 0 and self.symbol in ("N", "O"):
            self.H_donor = 1
        else:
            self.H_donor = 0