Define preimage, then level set as a preimage with a symbol #467

Ezra asked on #461 whether a level set should have a symbol, and whether every definition needs one.

Decision (daytb): a definition gets a symbol only when the term recurs in formulas. A level set recurs, so it gets one, defined by its most literal meaning: the preimage of the single value c.

Plan, in small PRs:

1. mds: a "Preimage" def with an example (this repo).
2. macros: a preimage macro, and a level-set macro defined from it, plus the notation-choices entry.
3. mds: use the level-set macro in the level set def and example.

No merges without Ezra's word.

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