Add an explicit "Greenland" designation in the compset name

[billsacks]

Before adding support for Antarctica, I'd like to lay the groundwork for having an explicit designator in the compset describing whether you're running with Greenland, Antarctica, both Greenland and Antarctica, or some other ice sheet(s). The first step will be adding explicit designation for Greenland.

I plan to do this in a backwards-compatible way, so the lack of explicit designation will be equivalent to specifying Greenland-only.

[billsacks]

For compset aliases I'm thinking something like:

• T1850Gg -> Greenland only
• T1850Ga -> Antarctica only
• T1850Gag or T1850Gga -> Greenland & Antarctica

etc.

@whlipscomb @gunterl @Katetc thoughts on that?

[billsacks]

Maybe T1850Gag rather than T1850Gga. The general rule would be: if multiple ice sheets, they appear in alphabetical order.

[whlipscomb]

@billsacks - This makes sense to me. Down the road, other potential ice sheets could be 'n' for Northern Hemisphere, or maybe 'l' for Laurentide and 'e' for Eurasian.