Implement `scs2e`

[GregoireHENRY]

let et = unsafe {
    let mut et = 0.0;
    let sclk_str = format!("{}.0", headers.data_field.time.seconds());
    let date = spice::cstr!(sclk_str);
    spice::c::scs2e_c(HERA_ID_SPICE, date, &mut et);
    et
};