Tiff: Add SubIFD tag for faster indexing

[nclack]

Apparently, tifffile will use the SubIFD tag to accelerate sparse operations on images in tiff files. See this gist.

We can:

1. Add the SubIFD tag to the first IFD.
2. It should get initialized to something empty.
3. During acquisition, we can keep track of the offsets to each written ifd.
4. When the file is closed, append the subifd array to the end of the file and update the original subifd tag at the beginning of the file.

We'll want to confirm:

1. The file is readable by typical readers using this scheme.
2. The file is usable even if that final IFD update failed for some reason.

[nclack]

writing a file with tifffile:

im=rand(512,512)
with tifffile.TiffWriter('out.tif',bigtiff=True) as tif:
        for i in range(10):
            tif.write(im,contiguous=True)

and inspecting the contents shows the image description tag as:

{"shape":[10,512,512]}

software tag is set to tifffile.py

No sign of subifds ...