code: def xtqx(self):
          
          if self.__xtqx is None:
              self.log("xtqx")
              self.__xtqx = self.jco.T * (self.obscov ** -1) * self.jco
              self.log("xtqx")
          return self.__xtqx
docs: get the normal matrix attribute. Create the attribute if
          it has not yet been created
  
          Returns
          -------
          xtqx : pyemu.Matrix