Specify multiple gridUnits?

As I understand it, pubsweet is intending to change the gridUnit variable to 6px? This has already happened in the eLife theme.

We also have XXS, XS, S, M, L, XL & XXL for "vertical spacing", which have been added to our theme's code

Just feeding info back up, in case others would like this type of sizing, rather than computing gridUnit * 4 manually in each place in the code.

Personally, I'm unsure about XXS, XS, etc as variable names & would be more inclined to go with something like 1, 2, etc as we do with headings - see this comment in the eLife issue but perhaps this is a popular convention?

Relates #4, #10 & #24 (closed)