Construct a ``CavityAnalytic`` instance from the Pynac lattice element

def from_dynacRepr(cls, pynacRepr):
        """
        Construct a ``CavityAnalytic`` instance from the Pynac lattice element
        """
        cavID = int(pynacRepr[1][0][0])
        xesln = float(pynacRepr[1][1][0])
        phase = float(pynacRepr[1][1][1])
        fieldReduction = float(pynacRepr[1][1][2])
        isec = int(pynacRepr[1][1][3])
        return cls(phase, fieldReduction, cavID, xesln, isec)