Add ability to generate line list using NIST network services

[tepickering]

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&current=20&convolve=gaussian&widthnum=0.01&userfile=&state=init

[tepickering]

it was noted that it would probably be better to use the strong line databases. they can be fairly easily web-scraped from https://physics.nist.gov/PhysRefData/Handbook/element_name.htm. for each element, there's a strong lines link that will lead you to ascii tables like https://physics.nist.gov/PhysRefData/Handbook/Tables/thoriumtable2_a.htm.

[b1quint]

@simontorres and I have bee working on something like that. From my experience, the strong lines approach may not be a good option because different elements have pages with different numbers of tables (compare Na with Th for example).

I found the NIST Form for looking up wavelength tables and did a quick-and-dirty method that retrieves the data using the CGI. I just did a pull request on that (#30).

[simontorres]

I do prefer strong lines because the output is already filtered, though it is possible to do it afterwards.