Changes to the Marlin firmware for Parallel Dual Extrusion

This repo has a version of Marlin that we adapted for our project on parallel / simultaneous dual extrusion. Details are in our SFF 2021 paper.

The files that have changed:

• Configuration.h, Line 573
• Marlin_main.cpp, Line 6476 - 6498
• Configuration_adv.h, Line 331