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.
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.