An option to output prunned spines individually

[mcyc]

It may be good to offer an option to output pruned spine individually on separate images, as opposed to putting them all in one image, like that implemented by following line:

crispy/crispy/pruning/Skeleton.py

Line 162 in 2141729

self.spines = np.sum(self.spines, axis=0)

However, it's also worth noting that the current pruning method means the individual gridded spines are unlikely to touch each other, a situation that can create confusion when they are all placed in a single image.