Replace included cytometry tube rack

[castillohair]

@KarlGerhardt, do we want to replace the cytometry tube racks included with the ones Evan used in his thesis? It seems that every time somebody has built new racks in our lab in the last few years, it has been Evan's design and not the ones in this repo.

The only minor inconvenient is that these have not been designed in SolidWorks, so they would stick out with respect to the other 3D printed parts in this repo. But I don't think this should get in the way of presenting improved parts.