plus minor edits and fixes

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

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.

added epsilon value to weights in the MergeMertens in order to avoid zero weights for pixels from uniformly filled areas of image