Automatic brain masking is too inclusive, add new method

[styner]

The current automatic brain masking is too inclusive for the purpose of entropy computation. Enable a new method that computes a restricted brain mask only comprising WM and GM excluding CSF for the purpose of entropy computation: 1- Compute IDWI as with FSL Bet on IDWI 2 - ImageMath -otsu 3 - ImageMath -conComp 1

This mask should be used ONLY for entropy computation.