Add concurrency/parallelism support to Dot operation for dense

Try to add concurrency/parallelism support for gnome/matrix package by using goroutine. Start with the Dot operation in dense_arithmetic.go. 

From my own experiment, I found the execution time for Dot operation can be reduced around 50-70% on 4-core machine.

I believe a lot of places of matrix package can benefit from goroutine concurrency. But just try first PR to get feedback from all you guys. If this is the right direction, I will continue to contribute to it.
Don't do this.