Group stripping lines by stream in drop-down menu

In the drop-down menu to select a stripping line, the options should be grouped by stream; and behind the scenes you're actually selecting a stream+line combination. This solves the minor issue of which TES location to choose when selecting a line that appears in multiple streams. It also gives a hint to the user of what sort of physics the line is for.

As an extension, we can render intuitive descriptions of each stream rather than the plain stream name.

This is achievable with the <optgroup> HTML tag.

Analogous to grouping countries by continent:

Where transcontinental countries appear under multiple continents.