Allow creation of empty mask

[valosekj]

This PR adds a feature that creates an empty mask if segmentation does not exist. This is useful, for example, when creating MS lesion segmentations from scratch.

Please, could you double-check especially the part of the code responsible for empty mask creation:

manual-correction/utils.py

Lines 254 to 264 in bba77ad

	def create_empty_mask(fname, fname_label):
	"""
	Create empty mask from reference image
	:param fname: path to reference image
	:param fname_label: path to output mask under derivatives
	"""
	img = nib.load(fname)
	data = np.zeros(img.shape)
	img_mask = nib.Nifti1Image(data, img.affine, img.header)
	nib.save(img_mask, fname_label)
	print("Empty mask created: {}".format(fname_label))

Fixes #4

[sandrinebedard]

Just need to add the added requirements, the rest works well!

I tested for FILES_LESION and FILES_LABEL and works perfectly.

[naga-karthik]

Minor suggestions, rest looks good!

[valosekj]

Thank you for reviewing! I addressed your comments. --> merging