MicroCube CoreXY conversion

I noticed some spare holes in the front corner parts of the original MicroCube printer design and instantly thought about a CoreXY conversion. I had trouble with the stiffness of the x carriage with the original H-Bot construction and this seemed to solve it.

Just print the new x carriage "x_carriage_coreXY_improved.stl" and the "x-carriage_tensioner_flipped.STL" for the other side. Also add 2 more U grooved bearings. Make sure one pair of bearings is above the other, so the lines don't rub