* 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* Remove outdated overloads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

e.g. <opencv2/core/core.hpp> become <opencv2/core.hpp>

Also renamed <opencv2/core/opengl_interop.hpp> to <opencv2/core/opengl.hpp>

Evangelidis, G.D. and Psarakis E.Z. "Parametric Image Alignment using Enhanced
Correlation Coefficient Maximization", IEEE Transactions on PAMI, vol. 32, no.
10, 2008

Made all STL usages explicit to be able automatically find all usages of
particular class or function.

* added DenseOpticalFlow interface
* moved OpticalFlowDual_TVL1 to src folder

This fixes bug #2075

removed rarely used fixed_size parameter from AutoBuffer type, added optional AutoBuffer* but to cvarrToMat in order to speedup CvSeq->Mat conversion; finished conversion of convex hull and related functions to C++

issue 2526

added recommended number of stripes to parallel_for_, modified some of the functions to use larger stripes (for better performance)

+ Improve performance of calcOpticalFlowSingleScale method
+ Small refactoring

Current results:

IMAGE NAMES          RMSE
Beanbags
Dimetrodon           0.329428
DogDance
Grove2               0.550852
Grove3               1.464699
Hydrangea            0.523277
MiniCooper
RubberWhale          0.367246
Urban2               2.717003
Urban3               4.185070
Venus                0.775422
Walking

Time (for Urban3):
17.490248 sec