revise implementation-notes.md

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Code review #52.

Deficiencies in implementation-notes.md:

(1) Some of the entries would be more appropriate as GitHub issues, since they describe unresolved issues (e.g. potential memory leaks)

(2) There is a heading for the "Racing Lab" screen, but no information.

(3) Not much of an "overview" is gleaned by reading this, which is the main purpose of the file.

Whoever finishes off this sim should revisit and revise this file.

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Also recommended to add a Terminology section to implementation-notes.md.

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Check list of things that implementation-notes.md should contain or mention for this sim:

• terminology
• there is no model-view transform (single coordinate frame)
• issues related to rounding and number of decimal places
• use of pattern for nested options

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Most complicated bits:

• markerEditorObserver` in DoubleNumberLineAccordionBox
• BagDragHandler and ShoppingItemDragHandler
• ShoppingItemContainer (bagRow, backItemRow, frontItemRow)
• ShoppingQuestionFactory
• Creation and management of bags and items

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• Terminology:
  • marker
  • marker editor
  • track marker
  • rate
  • numerator
  • denominator
  • item row, bag row, cell

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Model hierarchy for shopping screens:

• ShoppingItemData

• ShoppingScene (FruitScene, VegetableScene, CandyScene)

• ShoppingContainer (Shelf, Scale)

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Done, closing.