add_sum! cleanup

[jarlebring]

Considering that add_lincomb! now can take more than two terms, the function add_sum! seems a bit redundant. There seems to be an opportunity to remove code. I think we should

1. Go through all the uses of add_sum! and see if it can be changed to add_lincomb!
2. Consider removing the function add_sum
3. Consider adding a function expand_sum(graph,node): Take essentially the code in add_sum. It can take a :lincomb node and expands it as a sum each sum containing two terms.
4. Consider removing generators/bigraph.jl (which was needed when we only had two-term version of BLAS-calls)

Point 3 should only be done if we feel it is really needed. Less is more.

[jarlebring]

I'm closing this for now. Can be reopened if we need expand sum.