The phase diagram is generated in a reduced dimensional space
        (n_elements - 1). This function returns the coordinates in that space.
        These coordinates are compatible with the stored simplex objects.

def pd_coords(self, comp):
        """
        The phase diagram is generated in a reduced dimensional space
        (n_elements - 1). This function returns the coordinates in that space.
        These coordinates are compatible with the stored simplex objects.
        """
        if set(comp.elements).difference(self.elements):
            raise ValueError('{} has elements not in the phase diagram {}'
                             ''.format(comp, self.elements))
        return np.array(
            [comp.get_atomic_fraction(el) for el in self.elements[1:]])