Beta release 5.20

This release adds a bunch of new features. It also marks a change in the documentation. The "binary" file attached to this release contains the STL files and the text of the instructions. To view the instructions formatted nicely, with images, go to the documentation on github pages.

A brief summary of the important changes:

• A much stiffer Z axis, with less off-axis wobble when moving in Z
• A screw-on optics module that won't loosen with time
• The same microscope body for both low-res and high-res microscopes
• Viton O-rings are now easier to insert (for those of you who were put off by the bent paperclip tool, it's nothing like that! It now takes about one minute per axis if everything's going smoothly.
• Lower magnification for the lower resolution optics module (this is a good thing, I promise!)
• Ties to support wobbly parts during printing
• Better instructions
• Triangular "self tapping holes" to make it easier to put the screws in without damage.
• Numerous small bug-fixes.

We've been using this one for a while, and it is now ready for release. This commit should become the first beta release of the new version - hopefully it will be out of beta soon :) We'd love some people to road test this version for us!