Ensure fits Header subclasses are preserved on slicing and copying - #9229

The change looks reasonable, however I wonder if it's worth it to make Header subclass-safe. Was there any specific issue that requires this change or is it "just" to adhere to good-practices?

I'm really conservative if it comes to io.fits, and I wouldn't want to introduce API-changes that aren't motivated by some real issues/problems. What do you think @saimn ?

@MSeifert04 - I subclass Header myself, for a FITS-like header of radio baseband data. And mostly this works very well, but copying and slicing were exceptions (which I why I needed to override them: https://github.com/mhvk/baseband/blob/master/baseband/guppi/header.py#L92).

It is partially because the Header class is otherwise suitable for subclassing that I think it is worth to correct the few places where Header is hard-coded.

Ah, thanks for the examples. 👍

This looks good to me, subclassing Header can be useful and I see no reason why we shouldn't support this. I will let you approve and merge if you agree @MSeifert04 ;)

Approved, so I merge it now.

Thanks @mhvk!