Make small improvements re HFS lines

This branch contains a number of small improvements on HFS lines that I found necessary while working on a related paper.

It includes a new command save hyperfine populations, which I put together for the purposes of that paper. The command accepts a list of lines on input, but any line can be given, not just HFS lines. As such, it overlaps with the save lines populations command; they differ in terms of both the user interface and the output format.

The reason I allowed for a line list on input is that I did not want all HFS lines be reported; hyperfine.dat includes a number of neutral lines without any collisional data, which confuse the output.

There are several ways forward:

• Remove the neutral lines from hyperfine.dat, drop the input line list, and let the command operate on all HFS lines.
• Merge the two commands; e.g., allow for a second file name on save lines populations, adjust the output, etc; and drop the new command. This breaks backward compatibility.
• Do nothing more, let both commands exist.

Note that this branch was spawned from chianti10 branch. As such, I have made the relevant merge request a prerequisite to this.