added an exclusive member to nodes
saving work, things break
changed exclusive status to be queryed on nodes and added commodity to the node consturctor
updated the translator to use node exclusivity
made greedy solver use arc member functions
updated tests for new node-based exclusivity
made arcs first class citizens
updated this branch with the changes from the lpsolver branch
exclusive arc constraints are now determined exlicitly by exclusive node groups, where every arc for every node in the group is given included in an exclusivity constraint
added initial support for exclusive bids in order to model assemblies, etc.
added testing for exchange node group exclusivity
Added an additional check for a bid of different material
simplified arc constructor
made exclusive node adding a little more intuitive
updated arc docstring
fixed some previous rebase issues, all tests pass. @scopatz, we should be good to go.