Filament pipe holder

I've created a holder bar for my filament spools using a 50mm PVC pipe. It mounts below a shelf, resting on the shelf brackets with the `Holder` parts. This gives it just the right amount of spacing away from the wall and shelf. It's easy to move/lift the pipe to add/replace spools.

Because of the weight of the spools and the flexibility in the PVC pipe, I've created a mid span support which sits on a shelf from my desk. This is just a loose, moveable part that makes the entire thing very stable.

Since the pipe diameter is 50mm, printing can be done right from this holder. I use [this filament guide](https://www.thingiverse.com/thing:1657908) to make sure the filament enters the printer from right angle and doesn't scrape any sharp edges.