Trying to make SPP work

[IanButterworth]

SPP has route layers with 4 layers, and the routes overlap, which breaks the layer2out Dict.

### SPP ###
[maxpool]
stride=1
size=5

[route]
layers=-2

[maxpool]
stride=1
size=9

[route]
layers=-4

[maxpool]
stride=1
size=13

[route]
layers=-1,-3,-5,-6

### End SPP ###

See here for somewhat of an explanation: https://github.com/AlexeyAB/darknet/issues/2859