HDUList iterator

[bmatthieu3]

This PR makes API changes to make it more rust idiomatic. The fits object opened is an iterator over HDUs.

Here we loop over the image extensions only

use std::fs::File;
use std::io::Cursor;
use fitsrs::hdu::HDU;
use fitsrs::hdu::data::DataIter;
use fitsrs::fits::Fits;
use fitsrs::hdu::header::Xtension;

use std::io::{BufReader, Read};

let f = File::open("<your filename>").unwrap();
let reader = BufReader::new(f);

let mut hdu_list = Fits::from_reader(reader);

while let Some(Ok(hdu)) = hdu_list.next() {
    match hdu {
        // Iterator over the images only, discarding the other types of extensions
        HDU::XImage(hdu) => {
            // access the header
            let header = hdu.get_header();

            // access the data consuming the hdu object
            match hdu.get_data(&mut hdu_list) {
                DataIter::U8(it) => {
                    let data = it.collect::<Vec<_>>();
                    assert_eq!(num_pixels, data.len())
                },
                _ => todo!()
            }
        },
        _ => (),
    }
}

This PR also fixes UB from #5