Handling of COMMENT, HISTORY, END cards differs wrt cfitsio

[airnandez]

The returned value of Header.Keys() does not include cards of type END, COMMENT, HISTORY:

https://github.com/astrogo/fitsio/blob/0221d1a54f8c95dcfc14c3234c9091e3287ef411/header.go#L323

I understand that there is a method for retrieving the comments and history cards of a FITS header, I would suggest that for compatibility with cfitsio, those cards should be also be returned by Header.Keys().

In addition, it seems that astrogo/cfitsio assumes that in a given HDU there is only one card of type HISTORY or COMMENT. This is not the case with cfitsio.

In case it matters, for my tests I'm using cfitsio v3.370.

An example of a file with several COMMENT cards in the same HDU can be found at:

ftp://legacy.gsfc.nasa.gov/fits_info/sample_files/images/rosat_pspc_rdf2_3_bk1.fits