Raspberry Pi 2/3 Rackmount - Fixed

Update 27/07/2020:

- Added the rackmount fixed for Raspberry Pi 4
- Fixed the Raspberry Pi 4 Case: it's available both as a two parts print and as a single part, like the others.

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The original rackmount for Raspberry Pi 2/3 has a slight problem with the "sledge" mechanism: one side of the joint was asymmetrical (~ 0.25mm) thus preventing the case from smoothly sliding in.

The problem was exacerbated by printing at 0.3mm layer height and it became quite noticeable when using PETG (since the friction coefficient is quite high).

This is a fixed version, also with slightly enhanced tolerances for those who have yet to dial-in dimensional calibration on their printer.

Also, the screw hole at the bottom of the rackmount has been fixed: some slicers would not interpret correctly the hole and fill the last few layers.

There are two versions:

- rpi_b_2_3_-_v2 : has a slightly reduced body, so to