formula: catch min/max overflows at construction
For issue #485. * spot/tl/formula.cc, spot/tl/formula.hh: Catch min/max overflow when the operators are constructed. Also disable travial simplification rules that would create such overflow. For instance x[*200][*2] will not become x[*400] anymore. * python/spot/impl.i: Catch std::overflow_error. * tests/core/equals.test, tests/python/except.py: Add test cases.