[T] More potential features for mixing extruders

Filament sensors via I2C
- This would likely help greatly with the day-to-day dealings of using an 8-channel 3D printer.
- There's not enough pins on the Octopus Pro to be able to have a filament sensor for each input, but I hear that Marlin supports I2C communication and I've got some PCF8575 IO Expanders.
- This expander has 16 inputs, which means that I could implement a "low filament" and "no filament" sensor, which I will explain below.
Redundant channels
- I was thinking of a GCODE command to tell Marlin that it can switch to another channel if the filament runs out.
- There needs to be a way to differentiate that there isn't any filament (or plugging wire) into the hotend, so that the heatbreaks aren't clogged with material flowing into the wrong direction, and just that there isn't enough material to continue printing with it.
- Thus, the "no filament" sensor, mounted as close to the extruder inputs as possible, and a "low filament" sensor, which can be mounted a bit further away, is required.
- Unlike a switching extruder, mixing extruders can feed multiple channels in at once, meaning that there should also be a feature that actually just uses all redundant channels at once to increase flow rate.

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