Remove save_primary_header, inherit_primary_header attributes

[ejeschke]

This begins the removal of the save_primary_header and inherit_primary_header attributes and kwargs from AstroImage. The saving of the primary header is done when loading an image through the io_fits module and it is merged when include_primary_header==True in the AstroImage get_header() method.

The two kwargs issue a deprecation warning if used when creating an AstroImage.

[ejeschke]

@pllim, in line with your comments for deprecation of API on #978.

[ejeschke]

Rebased

[ejeschke]

@pllim, ok to merge?

[ejeschke]

Thanks, @pllim!