Cleanup my ops for NIFTy6 release
To prep for #296 (closed), move MatrixProductOperator to a separate file (violates the simple in simple_linear_operators) and remove SwitchSpacesOperator, as it is trivially reproducable by LinearEinsum.
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To prep for #296 (closed), move MatrixProductOperator to a separate file (violates the simple in simple_linear_operators) and remove SwitchSpacesOperator, as it is trivially reproducable by LinearEinsum.