Add and clean up layout files

[aappling-usgs]

Copying over https://github.com/USGS-VIZLAB/hurricane-maria/tree/master/layout is probably a good start. Also:

• look through the classes for opportunities to switch to more obvious, informative names (e.g., .treasure-mark and .gage-blocker in hurricane-svg.css may not be as clear to the next team as they were to the maria group). Propagate changes into the visualization scripts as needed.
• make sure elements are organized/grouped by the type of function they serve, and
• add comments where they would help future vizmeisters understand or locate the right element quickly.