Return of the digital Z stack.

[iandobbie]

So I did a whole load of work adding the digital Z stack which worked and was merged (see issue #691) but with the recent DeepSIM code I can see that this functionality was removed at some point. I think this also contributed to the deepSIM team re-inventing the wheel and leading to more divergence than might have been.

Why? This is was important extension to cockpit allowing system like the Zaber, or other digitally triggered system to do zStack without any analog synchronous control.

I am very keen to return this into the main branch so we can also work on options like the pi pico timing system.