WIP .hdr writer

[nilgoyette]

WIP because write_nifti is so ugly now that I didn't found the courage to make write_rgb_nifti work too. I would be really interested in your review. Like, how I can make this less ugly. Or even beautiful!

I added a lot of #[cfg(feature = "ndarray_volumes")] for is_hdr_file because it's currently only used with ndarray_volumes, but this will be removed "soon" when we support writing nifti-rs types.

[Enet4]

Well, in terms of what we currently have here, I don't happen to see anything egregiously bad or non-fancy. However, it would be nice to see how we can solve the major problems of writing files from here (as indicated in #19). Future versions should indeed become less and less independent from ndarray for writing.

[nilgoyette]

What I don't like is the

if
    if
        write
    else
        write
else
    if
        write
    else
        write

and the fact that the same logic will also be duplicated in write_rgb_nifti. I thought that maybe there was a better solution but it doesn't seem to be the case :) I'll make it work for rgb and update this PR.

[Enet4]

Well, I can't think of a way to simplify those if-else statements without moving those pieces into separate functions, which I don't think is necessary. However, a few other things to consider below.

[Enet4]

Good to go. I have a few other concerns that I wish to attend before making a new release, but we'll deal with those separately. Thanks!