feat(Dynamics/BirkhoffSum): birkhoffAverage const

[samueloettl]

---

I think this might be useful.
My feeling tells me that one of these should be a simp lemma.
I'm not really sure if I got the naming of the theorems correct.

When generalizing to the assumption (n : R) ≠ 0 instead of the special case CharZero R with n ≠ 0 I had to use "open Classical in". I'm a bit unfamiliar with that part so please check if this makes sense. See also #35307 (comment)

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PR summary 514b0e414e

Import changes for modified files

No significant changes to the import graph

Import changes for all files

Files	Import difference

---

Declarations diff

+ Function.const.birkhoffAverage_eq
+ Function.const.birkhoffAverage_eq'
+ Function.const.birkhoffAverage_eq₀
+ Function.const.birkhoffAverage_eq₀'

You can run this locally as follows

## summary with just the declaration names:
./scripts/declarations_diff.sh <optional_commit>

## more verbose report:
./scripts/declarations_diff.sh long <optional_commit>

The doc-module for scripts/declarations_diff.sh contains some details about this script.

---

No changes to technical debt.

You can run this locally as

./scripts/technical-debt-metrics.sh pr_summary

• The relative value is the weighted sum of the differences with weight given by the inverse of the current value of the statistic.
• The absolute value is the relative value divided by the total sum of the inverses of the current values (i.e. the weighted average of the differences).