Need to replace the use of "volumetric" with "volume_fraction" when referring to soil quantities

[nusbaume]

According to the standard name rules, the use of the term volume_fraction is preferred over volumetric. This means that standard names that are currently using volumetric_soil_X should be replaced with standard names that use volume_fraction_of_soil_X.

Also, assuming the fix for this comes in after PR #60 then it might also help to move all of these land surface standard names to be underneath the new land_surface section as well, as opposed to the GFS type(s) section.