Mulgen

as per #54 

Instead of at and bt I was thinking of switching it to tA and tB, to mirror the inputs to blas.Dgemm, and change the type from bool to blas.Transpose so that if we get complex matrices the interface will still work.  Alternatively, we could call it transA and transB to mirror netlib's quick reference at http://www.netlib.org/blas/

Or we could do something else, any suggestions?