Record data size in the WCS object

It is clear that there are practical reasons to have a record of the original data size in the WCS object. This in the past has been recorded in public attributes naxis1 and naxis2 and subsequently in the private _naxis1 and _naxis2. There’s along thread on why this should or should not be done in #4662. More recently #5411 expanded this attribute to more than 2 axes. It also broke the ability to set the private attributes which was subsequently fixed in #5454 as a stop gap measure.

We need to fix this permanently starting with v 1.3. There are two proposed ways to do it.

1. Add public naxisj attributes, populated by the FITS header NAXISj keywords.
2. Introduce a subclass of WCS which adds those and leave the original WCS to represent strictly the FITS WCS standard and nothing more.

Either way a deprecation process will accompany the change.

The purpose of this issue is to collect votes. I don’t want to start a new long discussion. Keep in mind that #4662 is available for reference and only share new arguments.

Since 1.3 is approaching we need to make a fast decision. So please share your preference here and volunteer for the work if possible.

[nden]

I support #1 of the two choices for reasons I mentioned in #5454.

In addition I want to mention that we already deviated from the FITS standard by adding the d2im distortion.

[mcara]

@MSeifert04 I think the “long discussion” was in https://github.com/astropy/astropy/issues/4669