Added reference for chunker and clean data.py

I swiped the chunker function in data.py from a stackoverflow question. Added
a link in case I need to look it up again.

Also did a very small bit of cleanup for the multiprocessing code, so that a
pool of processes is only generated if nproc > 1.
Modified the MSL file extraction

The MSL library file extraction did not work if there was not a trailing space
in the file. I fixed the code so that it wouldn't matter.
Fixed the calibration calc for internal std

I was not using the correct equation for determining concentrations from
internal standard calibration curves. It was a pretty small fix in terms of
lines of code, but a major calculation change.