Fixed some inconsistencies in interfaces of different white balance algorithms and improved
the documentation a bit. Also added an option of loading a model during runtime in the
learning-based white balance algorithm. Added a simple tutorial describing how to train a
white balance model.

Signed-off-by: Bartek Pawlik <bartek.pawlik@outlook.com>

Added an option to pass an initial approximation of optical flow in
DISOpticalFlow. Added a python sample that demonstrates the use of this
feature for temporal propagation of flow vectors.

minor refactoring

added more tests

minor

fixed warnings

minor

minor

minor

fixed comment

Implemented a learning-based color balance algorithm, which typically provides
noticeably better quality than grayworld, while being several times slower.
Added a python script for training this algorithm, as well as the benchmarking script
for objective quality evaluation.

Also made some modifications to the grayworld algorithm implementation, including the
16 bit image support and overall acceleration by about 1.7 times (for 8 bit images).

Moving more Caffe loaders to sparate file