get(direction=NOMINAL, names=ALL, diff=False, factor=False)
        Returns different representations of the contained value(s). *direction* should be any of
        *NOMINAL*, *UP* or *DOWN*. When not *NOMINAL*, *names* decides which uncertainties to take
        into account for the combination. When *diff* is *True*, only the unsigned, combined
        uncertainty is returned. When *False*, the nominal value plus or minus the uncertainty is
        returned. When *factor* is *True*, the ratio w.r.t. the nominal value is returned.

def get(self, direction=NOMINAL, names=ALL, diff=False, factor=False):
        """ get(direction=NOMINAL, names=ALL, diff=False, factor=False)
        Returns different representations of the contained value(s). *direction* should be any of
        *NOMINAL*, *UP* or *DOWN*. When not *NOMINAL*, *names* decides which uncertainties to take
        into account for the combination. When *diff* is *True*, only the unsigned, combined
        uncertainty is returned. When *False*, the nominal value plus or minus the uncertainty is
        returned. When *factor* is *True*, the ratio w.r.t. the nominal value is returned.
        """
        if direction == self.NOMINAL:
            value = self.nominal

        elif direction in (self.UP, self.DOWN):
            # find uncertainties to take into account
            if names == self.ALL:
                names = self.uncertainties.keys()
            else:
                names = make_list(names)
                if any(name not in self.uncertainties for name in names):
                    unknown = list(set(names) - set(self.uncertainties.keys()))
                    raise ValueError("unknown uncertainty name(s): {}".format(unknown))

            # calculate the combined uncertainty without correlation
            idx = int(direction == self.DOWN)
            uncs = [self.uncertainties[name][idx] for name in names]
            unc = sum(u**2. for u in uncs)**0.5

            # determine the output value
            if diff:
                value = unc
            elif direction == self.UP:
                value = self.nominal + unc
            else:
                value = self.nominal - unc

        else:
            raise ValueError("unknown direction: {}".format(direction))

        return value if not factor else value / self.nominal