Add ability to generate line list using NIST network services

NIST has a web application that lets you generate a thorium line list given a few parameters. the site can accessed at https://physics.nist.gov/cgi-bin/TH/site.cgi. pretty straightforward api for making the requests, e.g.:

https://physics.nist.gov/cgi-bin/TH/site.cgi?atlaschoice=Thorium&xscale=Wavelength&medium=vacuum&units=nm&low=350&high=360¤t=20&convolve=gaussian&widthnum=0.01&userfile=&state=init

Start by reproducing the example request against the NIST Thorium line-list service at https://physics.nist.gov/cgi-bin/TH/site.cgi and review its returned data. Determine where this repository should expose the request parameters and how the generated line list should be represented; done means a caller can request a thorium line list for specified bounds and settings.

Written by the indexing model from the issue text.