Doesn't handle .nii file

[chrisk1]

Running the segmentation on an uncompressed T2_brain.nii file gives the following error:

Cifstream::Open: Cannot open file T2/T2_brain.nii.gz
Error: convert-image command returned non-zero exit status 1

Converting the .nii to a compressed .nii.gz format works normally. An unimportant error check to add in perhaps!

[amakropoulos]

Thank you for spotting this :) !
This is now fixed in #21