fix AnalyticOptic.to_fits() for complex type

[mperrin]

Can't store complex numbers in FITS files natively so the following fails.

optic = optics.ThinLens(radius=1)
fitsfile = optic.to_fits(npix=512, what='complex')

Fix should probably be to pull out the real and imaginary parts, and save them as 2 slices of a data cube, with appropriate FITS headers to indicate such.

Leaving this for later, not release 0.4.1 immanently. Low priority.