Implement "sandwich product" matrix operations#798

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devinamatthews wants to merge 1 commit intomasterfrom

Commits on Apr 27, 2024

Implement A*D*B (gemdm), A*D*A^{T,H} (syrkd/herkd), and A*D*B^{T,H}+B*D*A^{T,H} (syrk2d/her2kd) operations (and gemdmt for good measure). Some rough edges still:
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