Implemented Division Protocols for Double Array

I had the time to look over and create PMatrixDivideMutable and PMatrixDivide for the double array. I tended towards performance with this implementation, which is why I try to avoid the mp/broadcast-compatible if the array is being divided by a number (opposed to another array).

Additionally, on the div!, I avoided using element-map to avoid the unnecessary copy and mp/assign!.

Let me know how this looks.