Bowed legs for c270 manual

Description updated with final versions

From an idea by @julianstirling.

The C270 camera casing is constrained in width because it needs to fit between the legs and still allow movement. Even with the casing the same width as the board the sideways motion is restricted to 3mm each way (6mm travel). It would be useful to allow the casing to be around the board so that it is not affected by changes to board components.

Making the legs bowed does not alter the motion geometry, but can keep them out of the way of the camera. This should allow the full 6mm each way, 12mm total motion. Might be enough space for full travel and another 1.5mm width on the casing to get the walls off the board (currently casing walls are 0.75mm thick). The top end of the legs are beginning to look weak.

This is tied to the Manual version because if a microscope is motorised it needs motor control etc, so there is a Pi and so a Pi camera is a better option as it is similar cost and resolution but better integrated.

Changes:

• Bow the legs of the Manual version so that the legs do not contact the C270 optics module
• Change the casing for the C270 camera - lens spacer and backshell - so that there is more space for different components that appear on various C270 boards. The shape is designed together with the bowed legs to maximise space for the camera board and while keeping the range of motion.
• Add a step to the build instructions to remove two of the ladder ties in the bowed legs. These ties are now shorter and do not form the same parallelograms as the rest of the leg, which will put a stress on the legs when tilted.