Read units from an acclaim skeleton file stream.

def read_units(self, fid):
        """Read units from an acclaim skeleton file stream."""
        lin = self.read_line(fid)                   
        while lin[0] != ':':
            parts = lin.split()
            if parts[0]=='mass':
                self.mass = float(parts[1])
            elif parts[0]=='length':
                self.length = float(parts[1])
            elif parts[0]=='angle':
                self.angle = parts[1]
            lin = self.read_line(fid)
        return lin