Fix bug: add new initial path only if not already existing
Add 'diverse' pricer type

The new pricer, 'diverse', maximizes the 1-norm of the difference
between the new solution sigma and an average of the already
existing paths.
For each arc, we count its 'average incidence value' (in [0,1])
as the number of paths that use such arc divided by the
total number of paths for (s,t).