Support states which are non default constructable about hsm

[erikzenker]

I am thinking about the problem and possible solutions. I don't think that returning the target object in the current actions is the most sophisticated solution to initialize the state. It will make the transition table harder to understand. Two other solutions that came into my mind:

1. Provide state objects to the state machine object.
   The problem here is that you might not have all parameters to construct the state yet.

2. Provide a special on_entry event as it is done in the sml
   The action of this event could return the constructed target state

   hsm::state<S1>{} + hsm::on_entry{} / [](auto event, auto source){ return S1{std::string{"Init string"}};}  

[erikzenker]

@BenjaminW3 can you give me more details on your use case?

[BenjaminW3]

In the use case I have in mind, the target state is constructed based on the members of the event leading to the transition to the to-be-constructed target state.
Using on_entry is probably already too late because there the event is already gone.

[BenjaminW3]

For reference the rest of my proposal from the other PR discussion:
"I would still allow void returns from actions to use the default constructor. Furthermore, we would have to make sure that only one of the actions in a transition (maybe only the last one?) may return the next state"

[erikzenker]

Just for the record: I found two bugs while writing the on_entry state initialization integration test (see #103 and #104)

[erikzenker]

#103 and #104 were fixed

[erikzenker]

Accidentally, I found out that the sml also supports data members. Maybe because it is not mentioned in the documentation I missed the feature until now.

Just for the record, the user can provide non default initialized states to the constructor of the state machine. I guess if he doesn't they will be default initialized.

The solution is like the first one I already discussed above. And it still has the problem that some states might need data from an event to be constructed.

[erikzenker]

I added a PR for the feature #118. Would be nice if you can have a look at it.

[erikzenker]

Merged with #118