Add a vips tilesource

[manthey]

This class can be used to write files as well. This is a new capability and should be regarded as somewhat in flux. Specifically, you can do

import large_image_source_vips

newsrc = large_image_source_vips.new()
for chunk, x, y in chunk_generator: # some process that generates numpy arrays or images
    newsrc.addTile(chunk, x, y)
newsrc.write('output.tiff')

[manthey]

Eventually, this should be generalized, so that there is a large_image.new(<some criterion>) method that would choose a sink based on some feature and could emit multiple file formats. There are TODOs listed in the source for this for other ways to extend it.