DMRECON: x.get() -> x != NULL in boolean context
DMRECON: check whether scale exists

Check whether there actually is an image for the given scale in
the image pyramid, throw an exception if there isn't.
DMRECON: make it work again on odd image sizes
DMRECON: catch exceptions inside DMRecon::start()

Better cancel one reconstruction than let the whole program fail.
DMRECON: assert on scaled image

getScaledImage() is only called on the reference view, which should
always have a scaled image available.
DMRECON: use mve::geom::pixel_3dpos