... or the pointer might be invalidated if the environments
changes.  Fixes #63.

* src/taalgos/dotty.cc, src/tgbaalgos/dotty.cc,
src/tgbaalgos/dtbasat.cc, src/tgbaalgos/dtgbasat.cc:
Copy the environment in strings instead.
* wrap/python/tests/automata.ipynb: Adjust comment.