Brainextraction

This pull request contains the following:

-medotsu and medotsu4D masking and cropping methods.
-Tests for the mask.py module.
-brainextraction_dwi example that uses medotsu.
-fetch_scil_b0 method in fetcher.py
-replace hardcoded threshold in reconst_csa.py and reconst_dti.py with medotsu.