Use only one model/cplex for each s-t

Use only one model/cplex instance for each s-t, no need to
delete/clear the model or the cplex instance and create it again.
IloCplex is updated automatically when the model is modified.
Moreover, a further improvement is expected by the fact that
we are re-optimizing over the same structure, so a warm start
(hopefully useful) should be performed.
Postpone addition of variables generated in primal heuristic

Generating variables in a primal heuristic it's not allowed
in SCIP (it doesn't seem to work well, anyway), so just
generate the new path (if necessary). It's up to the pricer
to add the necessary additional lambda variables
(check if the number of lambda is smaller than the number of
paths).
Merge branch 'cplex-pricing' of github.com:sbebo/MMF into cplex-pricing