FiniteStateMachine: implement final_word_out

[cheuberg]

Implement FSMState.final_word_out in order to accomodate subsequential transducers in this package. A transducer is said to be subsequential if it is deterministic, every state is final and it has a final output word for each final state, i.e., when reading an input and reaching some final state along some path, then the final output word of this state is appended to the output labels collected along the path.

This will facilitate various transducers, e.g., when we currently have to read a sufficiently large number of zeros in order to flush some more output.

final_word_out is introduced in #16253. Now final_word_out is implemented everywhere (or there is a NotImplementedError, see #16548).

[cheuberg]

I reviewed the patch. Builds, doctests pass. I added one commit with rather minor changes (punctuation, indentation, .. TODO:: tag) and moved the new example from the private method _composition_explorative_ to the public method composition.

Please cross-review these changes and feel free to set the status to positive_review.

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Last 10 new commits:

​13e000c	FSMState.copy also copies final_word_out
​bcf3308	Transducer.asymptotic_moments only accepts transducers with all states final and transducers.GrayCode is now such a transducer.
​8d29348	Correct TeX errors (raw string vs. string) in docstring (Gray Code)
​ae8ec6b	Modified a few docstrings to reflect use of final output word
​4927542	removed determine_alphabets=True from docstrings because it is the default anyway
​ba07d56	Use iterators instead of lists
​0be171e	Added examples for FiniteStateMachine.equivalence_classes
​95414f5	NotImplementedError in _composition_explorative_ if there are final word outs
​c3f2b33	corrected spacing
​d1f3e21	trac #16191: Reviewer Patch: Move one example; spacing; typo; punctuation

[dkrenn]

Reviewed your changes (and looked at changes of skropf as well). Fine for me.