Preprocessing to enhance the target boundaries

[wyli]

Is your feature request related to a problem? Please describe.
originally from https://github.com/btgraham/SparseConvNet-archived/blob/master/SparseConvNet/Data/kaggleDiabeticRetinopathy/preprocessImages.py

it's been used in several challenges such as Kaggle Diabetic Retinopathy Detection