This should fix that pesky test failure that pops up from time to time.

I don't actually know if the default values should be zeros, but the tests
pass, so...

Looks like we don't actually use them; when creating precompiled headers
with Visual C++, a dummy .cpp is created at build time.

ReST directive was used. Also fixed some other ReST directives that were
not correct and removed some warnings during buildbot checks.

- Used lunix style slashes - works on most systems
- Removed all trailing whitespaces

This should allow many algorithms to take advantage of more parallelization
technologies.

Also cv::, cv::gpu:: and cv::ocl:: namespace prefixes can be safely omitted
inside CV_ENUM and CV_FLAGS

* Drop some low level API
* Remove outdated overloads
* Utilize Input/OutputArray

Previously the pyramid was done with a rescaling factor of 2 (implied by the
use of pyrDown). This often leads to inferior results compared to a scale step
of e.g. 0.8 (a factor of 2 is obviously faster). This commit makes the scale
step configurable and uses a resonable default value.

The other change in this commit is that median filtering is added. This is not
described in this paper but it is done in the author's implementation. (See
e.g. "Secrets of optical flow estimation and their principles", Sun et al.,
CVPR 2010) This serves as periodic outlier removal during optimization, leading
to smoother flow fields while preserving motion edges. This includes splitting
the optimization loop into two loops.

This also fixes few clang build errors

initial optical flow was not cleaned during the second call

This makes arguments of type InputOutputArray required in python unless they
have a default value in C++.

As result following python functions changes signatures in non-trivial way:

* calcOpticalFlowFarneback
* calcOpticalFlowPyrLK
* calibrateCamera
* findContours
* findTransformECC
* floodFill
* kmeans
* PCACompute
* stereoCalibrate

And the following functions become return their modified inputs as a return
value:

* accumulate
* accumulateProduct
* accumulateSquare
* accumulateWeighted
* circle
* completeSymm
* cornerSubPix
* drawChessboardCorners
* drawContours
* drawDataMatrixCodes
* ellipse
* fillConvexPoly
* fillPoly
* filterSpeckles
* grabCut
* insertChannel
* line
* patchNaNs
* polylines
* randn
* randShuffle
* randu
* rectangle
* setIdentity
* updateMotionHistory
* validateDisparity
* watershed