3D switch block updates & commenting

[vaughnbetz]

We have some cleanup to do on the 3D switch block code.

1. move --custom_3d_sb_fanin_fanout to the architecture file as a new tag, rather than as a command line option. It should go in the section.

2. Add a big comment somewhere (maybe in rr_graph.cpp) about how custom_3d_sb_fanin_fanout works. It should detail exactly what the rr graph this generates looks like.

See #2370 for the original cleanup comments.