[ADD] Implement adjoint for sub-matrix linear operator

[f-dangel]

This is useful if we want to estimate the Frobenius norm of a sub-matrix B = P1 A P2 where P1, P2 are slicing matrices via Hutchinson trace-estimating Tr(B^T B). We then need to define how to take the adjoint of a SubmatrixLinearOperator.

[coveralls]

Pull Request Test Coverage Report for Build 9198493683

Details

• 3 of 3 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.02%) to 88.383%

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Totals
Change from base Build 9043296647:	0.02%
Covered Lines:	1263
Relevant Lines:	1429

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💛 - Coveralls