Add support for 3D Slicer

[valosekj]

@yw7 has implemented a nice script for manual SC segmentation corrections using 3D Slicer viewer (a good option for Windows users), link here.

Our manual_correction.py script has a TODO to add 3D Slicer support:

manual-correction/manual_correction.py

Lines 364 to 367 in b1cf43e

	# launch 3D Slicer
	elif viewer == 'slicer':
	if shutil.which('slicer') is not None:
	# TODO: Add instructions for 3D Slicer

--> We could take inspiration from Yehuda's script and implement 3D Slicer support to the manual_correction.py script.

Tagging @NathanMolinier and @plbenveniste since this feature might be relevant for them.