Modified 3D printed OpenTheremin v.3 case parts

Modified 4 of the original files and added one. Added an integral tripod mount ( just needs a 1/4" nut ) to the bottom plate, added extruded text to faceplate, modified case to allow inserting the Theremin into the case with the antennas attached, and modified the I/O plate to allow using a ground wire. Also added a 7.4mm button.