make .at independent of type and number of channels by creating following methods:
template void setMatValuePtr(cv::Mat& image, int row, int index, double value);
void setMatValuePtr(cv::Mat &image, int row, int index, double value);
template T getMatValuePtr(cv::Mat& image, int row, int index);
double getMatValuePtr(cv::Mat& image, int row, int index);