adds overloads for faster toImage and addLayerToImage

[rooshm]

Addresses discussion in issue #359.

Overload created by use of boolean isAlpha, with the faster path available for images without transparency, when isAlpha is true, the slow path is used.

static bool addLayerFromImage(const cv::Mat& image, const std::string& layer,
                                grid_map::GridMap& gridMap, bool isAlpha, const float lowerValue,
                                const float upperValue, const double alphaThreshold);
static bool toImage(const grid_map::GridMap& gridMap, const std::string& layer,
                      const int encoding, bool isAlpha, const float lowerValue, const float upperValue,
                      cv::Mat& image);