Added a writeFormat() method to Algorithm which must be called by the
write() method of derived classes.

Reverts "Set the default value of exposure_weight in createMergeMertens to 1.0" because it breaks source code compatibility and tests
This reverts commit dbfd6a3d.

MergeDebevec takes camera response function as an optional input
parameter. Despite being clearly marked as input, this matrix is
overwritten during processing.

This commit adds a temporary matrix to store the log response and avoid
modification of the input.

* MulSpectrums
* StereoConstantSpaceBP
* BruteForceNonLocalMeans

http://code.opencv.org/issues/4124

avoid possible problems with zero values in tone mapping algorithms (http://code.opencv.org/issues/4020)

significantly improved parallel non-local means by using granularity parameter in parallel_for_ loop. Because the algorithm deals with sliding sums, it's essential that each thread has enough work to do, otherwise the algorithm gets higher theoretical complexity and thus there is no speedup comparing to 1-thread code (at best).

Changed parameter type of fastNlMeansDenoising[Multi][Abs] from std::vector<float> to const std::vector<float>&