Is it a tray or a shelf!?

[julianstirling]

          ... we need to fix all the errant references to `tray` throughout the codebase. I think the "tray" naming convention came from me during the hackathon, but Wakoma uses the term "shelf", so we should fix the codebase to use that term. An example can be found [here](https://github.com/Wakoma/nimble/blob/master/mechanical/components/cadquery/tray_6in.py).

Originally posted by @jmwright in #58 (comment)

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The code base uses both tray and shelf. Tray in the codebase also appears to have two meanings. Some times it means the whole shelf. Sometimes the shelf is divided up into the "front plate" and the "tray". Where the front plate is both the actual front plate and also any of the base that sits between the legs, whereas the tray is everything that sits entirely behind the legs (except in some cases when certain code switches are off.. which adds to confusion).

I think we need to:

• Do some naming standardisation in the existing repo
• Make a little diagram to explain the naming
• Enforce naming when doing code review