Open mosaic tile, add check if "too far"

[cpoczatek]

This is low priority.

When right clicking on a mosaic we prompt with the name of the nrrd file for that individual tile. Right now we're transforming the click from image coordinates to tile/stage coordinates and choosing the closest one. When clicking on an empty region this gives the user a name for a tile near by, not "nothing" which would be more correct.

We should choose the closest one, bound by tile size. Though it's possible we don't a priori know the tile size, which might make it difficult. To reproduce, build mosaic as below and right click on empty regions of the image.

wget https://github.com/BWHCNI/workflow/raw/master/mosaic_nrrd/test_data/090708-2-1_{11,13}-4x10.nrrd
mosaic_nrrd -t fluid -o test.nrrd 090708-2-1_11-4x10.nrrd 090708-2-1_13-4x10.nrrd