For distortion code, copying HDU 0 to HDU 2 needs to update some keywords

[mperrin]

Attention @shanosborne. There appear to be some FITS header keywords that need to be set for extension keywords. I'm seeing the following problem when trying to save PSFs. This is on astropy 3.0.2 in case that makes any difference.

I think you may need to avoid the deepcopy of the header after appending the new HDU, in your changes to calc_psf. The HDUList.append() function is supposed to handle the necessary keywords for an Image extension HDU, but right now that's getting overwritten with a copy of the 0th extension primary HDU header. When you make the new Image HDU, use both the data and the header in that call. Then append that onto the PSF HDUList. Try that and see if the files will write to disk after that, please.

nrc = webbpsf.NIRCam()
psf = nrc.calc_psf(nlambda=1)
psf.writeto('test.fits')

---------------------------------------------------------------------------
VerifyError                               Traceback (most recent call last)
<ipython-input-3-d45cf959f106> in <module>()
      1 nrc = webbpsf.NIRCam()
      2 psf = nrc.calc_psf(nlambda=1)
----> 3 psf.writeto('test.fits')

~/software/miniconda3/envs/astroconda/lib/python3.6/site-packages/astropy/utils/decorators.py in wrapper(*args, **kwargs)
    486                         # one with the name of the new argument to the function
    487                         kwargs[new_name[i]] = value
--> 488             return function(*args, **kwargs)
    489 
    490         return wrapper

~/software/miniconda3/envs/astroconda/lib/python3.6/site-packages/astropy/io/fits/hdu/hdulist.py in writeto(self, fileobj, output_verify, overwrite, checksum)
    851             return
    852 
--> 853         self.verify(option=output_verify)
    854 
    855         # make sure the EXTEND keyword is there if there is extension

~/software/miniconda3/envs/astroconda/lib/python3.6/site-packages/astropy/io/fits/verify.py in verify(self, option)
    117                     warnings.warn(line, VerifyWarning)
    118             else:
--> 119                 raise VerifyError('\n' + '\n'.join(messages))
    120 
    121 

VerifyError: 
Verification reported errors:
HDU 2:
    'XTENSION' card does not exist.
    'PCOUNT' card does not exist.
    'GCOUNT' card does not exist.
    'PCOUNT' card does not exist.

[mperrin]

Fixed by #215