Magnetic Rack for 1.5mL Eppendorf Tubes

This is a remix of [Magnetic Rack for 8, 1.5mL Tubes by acadey](https://www.thingiverse.com/thing:79424)

# 2018
- Versions for 4x and 12x tubes added.
- Thicker long front upper side of the model.
As it seemed to be printed much thinner than in the model
(at least when printing in PET, 0.4mm nozzle, 0.1mm layer, UM2+, without supports)

# 2017
Sometimes, our tubes jumped out of the original rack by themselves.
To circumvent the problem, I increased the height by 4mm to move
the s