sentinel color grading

[lennart-rth]

Sometimes the Sentinel Images are quit dark. Especially in the northern hemisphere (>30°).
There is already some color grading going on in sentinel.py:white_balance(). This needs to be replaced with a sequence of "correction steps" , to auto brighten, auto contrast and auto strech (colorspace not image size) the image. A good start would be the ImageEnhance Module from Pillow.