Feature discussion: advanced ELS mode & threading wizard

[Funkenjaeger]

This is a discussion thread for advanced ELS features that I'm prototyping for RCP.
My approach is to introduce a new bar to the ELS page which is OPTIONAL and hidden until the user invokes it, rather than introducing a whole separate mode screen. The advanced ELS bar offers incremental added capabilities, from a simple automatic stop (for turning up to a shoulder) all the way up to a full assisted threading wizard as introduced in https://github.com/Pawcu/rotary-controller-python/tree/feature/AutomaticThreading

The escalation of features is:

1. Basic ELS mode (unchanged from stock ELS mode)
2. ELS w/ electronic stop (for turning up to a shoulder; user can manually reset carriage w/ half nut disengaged, then re-engage ELS for next pass)
3. ELS w/ e-stop and retract (same as 2, but adds button to retract to prescribed Z position for convenience)
4. Assisted threading wizard (conversational step-by-step mostly-automated threading cycle)

Basic ELS mode:
(Note that I have made several customizations relative to the stock RCP appearance)

Advanced ELS mode, no advanced functions enabled
Behavior: same as basic ELS mode

Advanced ELS mode, stop enabled:
Behavior: when user enables the leadscrew, it will automatically stop when it reaches the "Stop Z" position

Advanced ELS mode, retract feature also enabled:
Behavior: will move to "Retract Z" when user presses "Retract", using mode 1 with trapezoidal velocity

Advanced ELS mode, assisted threading wizard enabled:
Behavior: will walk the user through the multi-step assisted threading sequence, modeled after Pawcu's implementation

[Funkenjaeger]

I refactored the assisted threading code, mostly in the form of a proper state machine using the threading library.
By following established design patterns I think it's more maintainable, and in the process I was able to adapt it to support all the modes listed above without a lot of code duplication.

The state transition graph (auto-generated) is currently this:

[Funkenjaeger]

I've finished integrating the wizard functionality on branch automated-threading-scale-sync paired with my revised firmware and it's working well on the firmware emulator, integration on HW still needs to be done.

Leveraging one of the benefits of this firmware, the user is free to open and close the half-nut in order to manually move the carriage if desired, without losing sync to the thread pitch (even if cutting metric threads on an imperial leadscrew, etc). Retracting manually rather than electronically cuts the amount of touchscreen interactions in half during a multi-pass threading cycle.
It's a future objective of mine to add some additional logic to enable automatic start of the next cutting pass when certain conditions are met, which would enable a full multi-pass threading cycle to be completed without touching the touchscreen after the initial parameter configuration - very much like the workflow of traditional mechanical threading on a lathe, but better - but that's out of scope for now.