Function pio_write_pypsa_csv_folder 

#[unsafe(no_mangle)]
pub unsafe extern "C" fn pio_write_pypsa_csv_folder(
    net: *const PioNetwork,
    out_dir: *const c_char,
    warnbuf: *mut c_char,
    warnlen: usize,
    errbuf: *mut c_char,
    errlen: usize,
) -> i32

Write net as a PyPSA CSV folder at out_dir. Returns 0 on success and -1 on error (the message is written into errbuf), the same convention as the other fallible int returns in this ABI. Fidelity warnings, if any, are written \n-joined into warnbuf.