Refactor printing of array values
Add SCFVariables class
REBASEME: directed_edges will be used in other models as well
Rename exprs to reflect their meaning
Add option of testing specific models
Initial incomplete work on SCF
Add main SCF constraint

There is still a problem which causes infeasible solutions.
The artificial root transmits k goods

One per node in the spanning tree.
Merge branch 'working' into workingfk
Improved in-degree constraint for mtz and scf

in-degree should be exactly 1 if node is active, 0 if node is inactive
added previously unsaved changes
slight simplification of mtz and scf constraints concerning in/outdegree

only marginal reduction of runtime