Parametric USB microscope (or anything) mount for PVC

Yet another mount for [cheap USB microscopes](http://www.ebay.com/sch/i.html?_nkw=usb+microscope&_armrs=1&_from=&_ipg=). I didn't like the ones already posted, since they either involved a ton of printing, had very complex assembly with precise tolerances, or relied on the the scope holder that comes with it, which I found to be the most finicky part.

This design is very simple -- a coupler that grabs onto a 1" PVC shaft, with friction set by a bolt; this mates with a holder, also set by friction and a bolt. The scope is simply zip-tied to it, since it wasn't worth my time to design a perfectly intricate holder when two zipties and a half-tube do would the job well.

The PVC shaft itself can be mounted any number of ways -- I went with a 3-way coupler and a few other bits of PVC. Total cost for tube and 3-way coupler was under $5.

It's almost fully parameterized, but I didn't enable customizer since I didn't have time to clean up the code.