- check boundary strictly
 - initialize the variable before using it

- Fix StereoBM ROI (Region of interest) selection for stereo
images used by block matching algorithm

The docstring for one of the Python sample programs includes a link to the research paper describing the main algorithm. That link is no longer valid (results in a 404 error) so this update replaces it with another link from the same institution which is currently valid.

- more aggressive cache for files under CMAKE_BINARY_CACHE
  * don't re-read files MD5
  * don't repack archives
- add support for ENV{OPENCV_DOWNLOAD_PATH}
- added ID parameter as project/group identifier
- non-flat .cache directory (based on ID)
- download message prefix based on ID
- more detailed logging via ocv_download_log() macro
- force .gitignore file for .cache folder (with '*' pattern)

AffineBasedEstimator crashed when called with an existing CameraParameters.
This happens e.g. when using Stitcher in SCANS mode.
CameraraParameters is now cleared before any calculation is executed.