Added preliminary functions to set negative weights

Added:
partition-sortby
competing-mapnode-fams
competing-mapnode-idx-fams
add-neg-wts-to-mat!

These are preliminary versions.  Need testing and analysis to make
sure they're actually doing what's required.

Also, I'd like to refactor add-pos-wts-to-mat! and rename
add-neg-wts-to-mat! so that adding both positive and negative weights
just use the latter.  It captures what's common to both processes.