Improve tile compressed pixel access API

[bmatthieu3]

Linked to #37

BinaryTableData is an enum that contains 2 variants:

• The TableData containing tabular data
• The TileCompressed variant containing Pixels data. Pixels is an enum that can contain U8, I16, I32, F32 uncompressed pixels. You get an iterator on the uncompressed native u8, i16, i32, f32 pixel values.

@Fingel - here is a api I achieved to provide. There is still a match to do because binary table can also contain real tabular data and not only tile compressed pixel data (haha). For such tabular data (the BinaryTableData::TableData), a pixels method has no sense.

 let img_bytes = match hdu_list.get_data(&hdu) {
    BinaryTableData::TileCompressed(Pixels::I16(pixels)) => pixels
        .map(|v| (((v as f32) / vmax) * 255.0) as u8)
        .collect::<Vec<_>>(),
    BinaryTableData::TileCompressed(Pixels::I32(pixels)) => pixels
        .map(|v| (((v as f32) / vmax) * 255.0) as u8)
        .collect::<Vec<_>>(),
    BinaryTableData::TileCompressed(Pixels::F32(pixels)) => pixels
        .map(|v| ((v / vmax) * 255.0) as u8)
        .collect::<Vec<_>>(),
    BinaryTableData::TableData(data) => unreachable!(),
};

I do not achieve big performance improving. Still a ~10% speedup which is reasonable but I would have thought I would gain more.

[Fingel]

This looks good! I will test this out tonight once the kid is asleep. I like the API.

[bmatthieu3]

@Fingel - Ok nice, if it is ok for you I can merge it.

[Fingel]

It works great!