BUG: Actual number of threads may be less that GetNumberOfThreads() if requested region has nb lines < GetNumberOfThreads()

BUG: Fix trailing whitespace trimming in otbApplicationLauncherCommandLine (kindly provided by Laurentiu Nicola)

std::string::find_last_not_of() returns a position, while std::string::substr takes an offset and a length. Fix the length computation and also add a comment regarding whether cleanStart must be checked.