This avoid compiler errors about ambiguous calls and make sure we are
calling the intended algorithms everywhere (this wasn't the case).

* src/tgba/tgbaproduct.hh (product, product_at): Rename as...
(otf_product, otf_product_at): ... this.
* iface/ltsmin/modelcheck.cc, src/bin/autfilt.cc, src/bin/ltlfilt.cc,
src/tgbaalgos/minimize.cc, src/tgbaalgos/powerset.cc,
src/tgbatest/ltl2tgba.cc, src/tgbatest/checkpsl.cc,
src/tgbatest/complementation.cc, src/tgbatest/ltlprod.cc: Adjust
callers.