Median otsu update

Increased the power of median_otsu masking by allowing to include non S0 volumes to the mask generation which is very helpful for masking hard brain or animal data (as discussed with @matthieudumont). I have also added an option for binary dilation for the cases were the mask was too aggressive near the cortex. This PR especially benefits masking the stanford data which we found difficult to segment with the previous version of median_otsu.