Parallelize implementation of HDR MergeMertens.
* Parallelize MergeMertens.
* Added performance tests for HDR.
* Ran clang-format.
* Optimizations.
* Fix data path for Windows.
* Remove compiiation warning on Windows.
* Remove clang-format for existing file.
* Addressing reviewer comments.
* Ensure correct summation order.
* Add test for determinism.
* Move result pyramid into sync struct.
* Reuse sync for first loop as well.
* Use OpenCV's threading primitives.
* Remove cout.
