Multithreaded keogram processing.
Multithreaded keogram processing.

Inspired by @sebmueller. This splits up the input images into batches of work
more or less evenly divided across the number of processors.

Two new flags are added (-m and -n were already used):
* -q / --level to make the workers be nice. default = 10
* -Q / --max-threads to control the number of worker threads. default = ncpus

Generates a keogram on my laptop in under 3.5s, even after doing horrible string
manipulations to compute time of exposure for label generation. Tested on rpi3b
too.

thomasjacquin/allsky,thomasjacquin/allsky,thomasjacquin/allsky