use annoy to get nearest neighbours for image similarity as well

[davidschlangen]

At the moment, the image similarity uses a homegrown, not very great way to keep the nearest neighbour indexing manageable. (In Preproc/sim_preproc.py.)

This should be using annoy, just as I later did in Preproc/CapEmbed/embed_captions_local.ipynb.

This however would mean that this needs to be changed in all notebooks in sempix that make use of image similarities, which may be quite a few.