Update MxArray.hpp

added contructor MxArray(const cv::Vec<T,N>& v)
can be used like this:
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cv::vector<cv::Vec<double,2> > outarray(2);
outarray[0][0] =22;
outarray[0][1] =23;
outarray[1][0] =24;
outarray[1][1] =25;
// Convert cv::Mat to mxArray
plhs[0] = MxArray(outarray);
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