FITS files do not contain NEXTEND

[braingram]

NEXTEND is listed in the keyword dictionary.

It is also not a standard fits keyword and doesn't appear to be produced by astropy.

I don't see how this could be produced by adding it to a schema since it contains a count of the number of extensions in the file.

Supporting this would require changes to stdatamodels when it produces the hdulist. After the hdulist is produced (and the count is determined) the NEXTEND keyword could be filled in.

Alternatively this could be dropped from the keyword dictionary.