update multiply() to gemm()

cc @bwlewis

I haven't tested this because I don't have a scidb installation with dense linear algebra working, but this is the change that needs to happen.

We still need to add spgemm support, but I didn't see the specification for this operator in the SciDB 13.12 manual.

Also, I'm not sure about the matrix chunk and type requirements for this. Do we want to copy the data in the background if the function call will fail?

Another thing to think about is whether we want to support previous versions where ``multiply()`` is available. If not, this change could break people's existing code.