Build the header (contain the number of CPU).

        CPU0       CPU1       CPU2       CPU3
        0:         21          0          0          0   IO-APIC   2-edge      timer

def __header(self, line):
        """Build the header (contain the number of CPU).

        CPU0       CPU1       CPU2       CPU3
        0:         21          0          0          0   IO-APIC   2-edge      timer
        """
        self.cpu_number = len(line.split())
        return self.cpu_number