refactor: Split LinearPMap from Unbounded

[gloges]

Splits off general results about LinearPMap from Operators/Unbounded.lean (sections which do not rely on an inner product structure) into the mathematics folder.
The individual commits walk through the split and subsequent clean-up in a way that I hope is easy to follow.

[github-actions]

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[jstoobysmith]

This seems a sensible separation - approved.