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.
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