New illumination dovetail

As discussed on the forum the illumination mount needs some improvement. Problems with the status quo are:

• The illumination is easy to misalign if knocked
• The plastic dovetail loosens over time and the condenser slides down into the sample
• The condenser interferes with the sample clips in the default version of the stage.
• The illumination cable waves around, which is unreliable and ugly

We'd like a solution that:

• Slides up and down, and friction-locks in place for easy adjustment
• Can be locked in place, ideally with one screw, once it's adjusted - ideally with minimal change in position.
• Makes it possible to add in nice cable management
• Self-centres to get as close to "aligned" as possible without too much tweaking
• Separates XY and Z adjustment of the condenser

Additional nice-to-have features include:

• Ability to add/remove sliders from the dovetail at any point in the track (i.e. without sliding them in from the end)
• Minimise breakable parts, and make sure breakable parts are easy to replace
• Make the mechanism reliable enough to use for the optics module as well, in the future

The version of the dovetail in this MR tries to incorporate a bunch of the features that have been suggested on the forum, and I think it ticks most of the boxes. It will need some iteration before it's fully ready, and it's currently unfinished.

Steps that are required:

• Convert my separate repo to use the libraries and dictionary convention in this repo
• Strengthen the locking flange
• Fix the flexure cut out in the internal bridge (thanks Julian for spotting these)
• Integrate the female dovetail with the illumination post
• Integrate the male dovetail with the condenser

Additional steps that would be nice but aren't required

• Add in cable management to the illumination post
• Add in cable management to the condenser
• Experiment with XY adjustment options (this may be a separate MR in the future)