Fix composite.py for Lasco images

[mgalloy]

Running composite.py with:

composite.py \
    --kcor-filename data/kcor/20141014_174849_kcor_l2.fts.gz \
    --kcor-radius 2.5 \
    --lasco-filename data/lasco/25521925.fts \
    --width=12 \
    --height=12 \
    2014-10-14T19:47:50

encounters an error:

...
    raise ConvertError("The source observer is set to None, but the transformation requires "
astropy.coordinates.errors.ConvertError: The source observer is set to None, but the transformation requires the source observer to be specified, as the destination observer is set to <HeliographicStonyhurst Coordinate (obstime=2014-10-14T19:48:00.340, rsun=696000.0 km): (lon, lat, radius) in (deg, deg, m)
    (0.00032415, 5.91325322, 1.49168362e+11)>.