Removed an unused import.
Shifted all the modules from lib.software to do with peak lists to lib.spectrum.

This is for a more logical organisation, as these modules are solely used by the
lib.spectrum.peak_list module.
Renamed all of read_*() functions of the lib.spectrum modules for consistency.

These functions will now be used to read all types of data from a peak list, from the assignments
to chemical shifts to peak intensites, and everything in between.