Allow to use '+' and '-' signs in front of the layer numbers in the active_layers option.
If no sign is present a fake stub will be used if needed.
If '+' sign is present a fake stub will never be used on this layer.
If '-' sign is present a fake stub will always be used on this layer (even if a real stub existed)

Ex : 5 6 7 8 +9 18 -19 -20 and maxFakeSStrips=2
-> Only tracks with a stub on layer 9 will create a pattern and layers 19 and 20 are desactivated (always a fake stub)