CarpetX: Interpolation with z-reflection symmetry only works for Psi4

[acwen11]

From the function CarpetX_Interpolate in interpolate.cxx:

if (reflection_z) {
    // The code below is only valid for Psi4
    assert(nvars == 2);
    assert(varinds[0] == CCTK_VarIndex("Weyl::Psi4re"));
    assert(varinds[1] == CCTK_VarIndex("Weyl::Psi4im"));
    // l^a = et^a + er^a
    // n^a = et^a - er^a
    // m^a = etheta^a + i ephi^a
    // Psi4 = C_abcd m-bar^b n^b m-bar^c n^d
    for (int n = 0; n < npoints; ++n) {
        if (symmetry_reflected_z[n]) {
            resultptrs[0][n] = -resultptrs[0][n];
            resultptrs[1][n] = +resultptrs[1][n];
        }
    }
}