SparseVector should support configurable defaults

[dlwh]

@dhgarrette's PR reminds me that SparseVector once supported (long long ago in old Scalala) default values other than zero. It was a bit hard to deal with, but it was worth it for log domain stuff.

Basically, SparseVector should have a default value variable that can be adjusted as it is updated by various operations.

[dlwh]

@gabeos this is along the lines of what you wanted to do. Just wanted to draw your attention to it.

[gabeos]

thanks, i think this'd be a pretty good next project, although it will definitely be a bit slower than the algebra work, since i have to refocus on my own research a bit. I'll take a look though, and think through all the changes that'd need to be made so i can start checking them off.

[dlwh]

totally. research comes first. Not trying to give you a job exactly, just in case you were interested. :-)