Minimalistic Mk8 Replacements

There's a few of these designs around, and while maybe veterans may already know, the purpose wasn't obvious to me at first so here's my attempt to explain the parts, function and reasons for this thing:-

* Firstly, the references to MK7 and MK8 are two versions of the same part - i.e. the top section of the replicator extrusion equipment.

* The stock Makerbot Replicator has a stepper motor that controls the feedrate of filament into the heater element below it.

* Pushing filament up a