Add implementations for Mat::zeros(int ndims, const int* size, int type) and Mat::ones(int ndims, const int* size, int type)

In class Mat, "zeros(int ndims, const int* sz, int type)" and "ones(int ndims, const int* sz, int
type)" are declared but never implemented. That means we can see their manuals from "http://docs.opencv.org/" but we can't use them. Here I tried to finish their implementation.

I have also changed MatOp_Initializer::assign to make it support multi-dimension Mat.

Test cases are added in test_math.cpp as well.