Add ability to compute level populations from lookup tables

[wtbarnes]

Starting in v10 of CHIANTI (and maybe earlier?), lookup tables were distributed with the database to avoid having to repeatedly compute the level populations and instead just do a lookup from a file.

There should be an additional underlying method that computes the level populations from these tables if they are available, perhaps controlled by some keyword. This will also mean parsing the lookup tables into the HDF5 version of the database.

Most notably, this would provide us a way to support later versions of the database without having to first fully work out the new ways of computing the level populations that are currently blocking this (e.g. #234).