Support tolerance technique for BellmanFord (#51)

A new operation traits class BellmanFordToleranceOperationTraits
is introduced, which uses the tolerance technique in its less()
function. This class can be used with the SetOperationTraits
named template parameter.

 EXTRA_DIST += \

 	scripts/bib2dox.py \

 	scripts/bootstrap.sh \

 	scripts/chg-len.py \

 	scripts/mk-release.sh \

 	scripts/unify-sources.sh \

 	scripts/valgrind-wrapper.sh