Smart Spool Holder

**UPDATE**
I've added a new rev. B: check below for improvements.

I have searched and tested many spool holders, but none actually fit my requirements so I made my own using designs I've found here as a reference, in particular:

* [this design](https://www.thingiverse.com/thing:1319713) from [ali55](https://www.thingiverse.com/ali55)
* [this deisgn](https://www.thingiverse.com/thing:1725851) made by [steveyum](https://www.thingiverse.com/steveyum)

This spool holder is meant to stand on top of an `8mm` thick frame, like the _Geeetech I3 Pro B_ acrylic frame, and is capable to withstand the weight of up to `3` 1Kg spools (around `3.6Kg` total weight).
It is intended to optimize the volumetric impact of the printer, not extending too much vertically while remaining within the printer horizontal projection area. The curved lines of the holder allow some flex under weight, but they also ensure no interference with the LCD and its buzzer.
Used with the _standard_ 1Kg spools (`