Removed runtime out-of-bounds check and added asserts where OpenCV doesn't check already
Removed runtime out-of-bounds check and added asserts where OpenCV doesn't check already

Notes:
* Removed the runtime-check in get_mean_at_point()
* I couldn't find out whether ::rowRange has a bounds check in debug builds. Therefore, added an assert.

icyrizard/eos,patrikhuber/eos,icyrizard/eos,icyrizard/eos,patrikhuber/eos,patrikhuber/eos