Refactored accumulator to allow it to accumulated and append multiple attribute/values to vertices
Implemented MV vs Demand sequence model, tested...seems to work
Merge pull request #24 from SEL-Columbia/output_shape

Output shape
completed removal of nodes slot from NetworkPlan.  Merged
addressed minor issues in write.NetworkPlan (works now with attributes :) and add sequence_models
make nodal outputs more consistent with existing rollout and select subnet roots via max demand
Merge pull request #27 from SEL-Columbia/cleanup

make nodal outputs more consistent with existing rollout and select subnet roots via max demand
Merge branch 'master' of github.com:SEL-Columbia/networkplanner.R into download
polish up download funcion and debugging