outfile option doesn't work properly for the calc_datacube function

[obi-wan76]

The outfile doesn't save the results from the calc_datacube function as a cube but as a PSF map. This comes from a Webbpsf helpdesk ticket by Matilde Mingozzi , INC0205772.
In the meantime users can save the correct file with the write method outside calc_datacube. E.g., :

cube = miri.calc_datacube(waves)
cube.write('filename.fits')

[mperrin]

@BradleySappington FYI I just re-noticed this webbpsf issue, which is actually a poppy issue. I'll make another tiny PR to get this one too, momentarily

[BradleySappington]

thank you!