Rename layer group "Median extent" -> "Median extent (1981-2010)"

[trey-stafford]

In the "Sea ice" root group. This is already in the layer description but we think it should be more clear from the layer group title.

[trey-stafford]

This reminds me that we have a problem with layer titles being ambigious without the context of the layer groups. Many tools in QGIS only display layer titles in dialog boxes, so a user is stuck with trying to figure out what e.g., a layer titled "January" is (which is a layer under the "Median extent" layer group, which is itself under "Sea ice")