add layer_strides() vector/accessor to net.{cpp,h}

... and use it to compute sbin / convnet_subsampling_ratio in get_sbin() in featpyra_common.hpp
weaken too-strong assertion in PyramidStitcher.cpp on {x,y}Max versus plane size
rename convnet_subsampling_ratio -> sbin
use get_sbin(net_) to get sbin in pycaffe and matcaffe.

but, TODO/FIXME the hard-coded usage in Patchwork.cpp