feat(Analysis/InnerProductSpace): a linear map composed with its adjoint is symmetric

[nielsvoss]

Add theorems about LinearMap.IsSymmetric that match some theorems for LinearMap.IsPositive.
Mathlib contains stronger versions of some of these, like LinearMap.IsPositive.self_comp_adjoint, but having a dedicated version for LinearMap.IsSymmetric is advantageous, especially when used with LinearMap.IsSymmetric.eigenvalues.

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One thing that I am uncertain about is if I should add similar lemmas for ContinuousLinearMap to even better match the IsPositive API, and if so, whether these theorems should be stated in terms of IsSymmetric or IsSelfAdjoint.

[github-actions]

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[github-actions]

PR summary 4aa0616ac5

Import changes for modified files

No significant changes to the import graph

Import changes for all files

Files	Import difference

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Declarations diff

+ IsSymmetric.adjoint_conj
+ IsSymmetric.conj_adjoint
+ isSymmetric_adjoint_comp_self
+ isSymmetric_self_comp_adjoint

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 script/declarations_diff.sh contains some details about this script.

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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).