Convert border_masks.m / IdentifyLargeBorder.m

[LeeKamentsky]

see #5

I ran the pipeline on the AC3 dataset. This script generates files of the form, outer_mask_NNNN.png and border_mask_NNNN.png. After running the script, every pixel of every file was 0.

So I am not sure if this is working - we should document what the code does before replicating the method exactly.