A distinct type for output register automata

[mtf90]

Currently, RALib supports the concept of outputs via a special type of OutputTransition. However, as far as I can tell, the concerned register automata are still acceptors and the transduction is actually implemented as a formal language of alternating input symbols and output symbols (so a transduction i_1 i_2 -> o_1 o_2 is represented as an (accepted) word i_1 o_1 i_2 o_2).

Would it be possible/reasonable to introduce a separate type (e.g., a RegisterMealyMachine) that actually supports transition outputs directly? This could be interesting from a performance point of view (requiring fewer locations to represent the same transduction) and could de-clutter some of the code by getting rid of "ioMode" checks which could instead be encapsulated in separate, type-specific implementations.

[mtf90]

The main issue I have with the current implementation is that a lot of code uses instanceof checks to differentiate between "input" and "output" mode (e.g., doing additional steps in case of OutputTransitions and OutputSymbols). This forces users to know of the specific sub-types and more importantly use them, because otherwise the instanceof checks fail with custom implementations. Furthermore, any problems would only be detected at runtime, because the current RegisterAutomaton only enforces a rather generic Transition type. With a more sophisticated type distinction (e.g., requiring OutputTransitions explicitly on RegisterMealyMachines) this would add more flexibility and compile-time type safety to the implementations.

Given that a lot of the code seems to be based on the work of [0,1], I assume @fhowar would be the best to comment on this. As far as I can tell, there is no inherent downside to modeling (instantiated) outputs as transition outputs rather than "special" transition inputs. Quite the contrary: we may get rid of some FIXMEs and have more consistent semantics.

I have added some initial drafts which also include some of the view separation discussed in #70.

[0] - https://link.springer.com/chapter/10.1007/978-3-642-34026-0_41
[1] - https://www.faculty.ece.vt.edu/chaowang/difts2015/papers/paper_5.pdf