Add `trace`, `det`, `logdet`, and `frobenius_norm` property to `KFACLinearOperator`

[runame]

Resolves #96.

Please merge #91 before reviewing this PR.

[coveralls]

Pull Request Test Coverage Report for Build 8383381257

Details

• 68 of 74 (91.89%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.1%) to 89.513%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
curvlinops/kfac.py	68	74	91.89%
<!--	Total:	68	74	91.89%	-->

Totals
Change from base Build 8364730614:	0.1%
Covered Lines:	1084
Relevant Lines:	1211

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💛 - Coveralls

[runame]

No, the main reason for using properties here is the caching and that it fits semantically (they are attributes/properties of the matrix). I think the resetting is a small price to pay for being able to cache the result, especially since we just have to add one new line in the resetting method for every additional property we implement, so seems easy to extend.