fixed s2MergeByTime bug, backwards compatible

[EtienneKras]

added functionality (backwards compatible) to filter duplicates using s2merge = 'dd' in the complete compute_inverse_depth function or s2TimeMerger = 'dd' in the getImages subfunction. Note that dd stands for the capping on days in the mosaicbyTime function

@Jaapel @gena, please provide feedback on whether the coding (formatting and quality) is in line with the repository and whether you agree with the changes. I checked the changes locally before opening this pull request. There were no crashes and everything worked fine on a test case in Taiwan. Original code did not changed as it was added using **args (backwards compatible).