Add sink debugging snapshot for the HomogeneousBox example

I'm adding the snapshot I've always used for sink debugging. This can be useful for anyone who wants/needs to debug sinks and other hydro/stars/sinks interactions. It's a level 8 (N_part = 2^(3*level)) box at a more advanced stage of collapse but before sinks/stars form.

@matthieu, the link to download the snapshot is here. Once you've uploaded it, I'll update the getDebuggingICs.sh script.