Implement sum(..) and sum_and(..).
Fixes #231. * NEWS: Mention of implementation of sum, sum_and. * bin/autfilt.cc: Add --sum, --sum-or and --sum-and options. * python/spot/impl.i: Add bindings for sum, sum_and. * spot/twaalgos/Makefile.am: Add sum.cc, sum.hh. * spot/twaalgos/sum.cc: Implement sum, sum_and. * spot/twaalgos/sum.hh: Declaration of sum, sum_and. * tests/Makefile.am: Add sum tests. * tests/core/explsum.test: Test the sum of two automatons, false or false, unsatisfied mark propagation, handling of univ. transitions. * tests/python/sum.py: Check that two automatons that does not share their bdd dict are not accepted, then run tests over the sum of randomly generated LTL formulas.