Added `seems-to-work' elastic montaging. Missing parts are
* handling disconnected graphs, the `after-SIFT-montage' situation is
used as is to establish block matches
* there is not yet a `multi-layer mosaic' which would help connecting
disconnected graphs and help stabilizing the whole thing
* not yet tested if it works with coordinate transformed images (it
should, but who knows before testing...)
* montaging of a series of layers is not yet accessibel through the GUI,
AlignTask is still a terrible mess
* annotation transfer (vector data) is not yet done