Move dotblas to multiarray

Preliminary PR moving dotblas_matrixmultiply down into multiarray. It is not quite complete as the `inner` funtion should also be done, along with the new `__matmult__` method for Python 3.5. However, this lets me test current functionality and provide an opportunity for review. If it seems reasonable to merge this, I can make new PRs to finish the job.
Perhaps I can cherry pick the relevant commits.