Tpetra::Experimental::GEMM: Fix for mode = "C"(onjugate Transpose)

[mhoemmen]

@trilinos/tpetra @amklinv

GEMM currently only implements the Non-Transpose ("N") and Transpose ("T") modes, not the Conjugate Transpose ("C") mode. The GEMM interface has no way to return an error for "not implemented," and can't throw an exception, so we unfortunately do have to implement all the options.

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