Changelog

`20220211.0`

Added ReturnThread. Unrelated to parallel mapping but a useful tool to allow multiple functions to run at the same time.
Fixed bug when number of items is smaller than chunksize
Added total keyword to pre-specify the size of seq if it's known. (e.g. itertools.combinations does not know the size but you can compute it)
tqdm fallback also prints to stderr
Updated documenation