New standard names for land surface variables

[SamanthaPullen]

I would like to add some variable names to the CCPP Standard Names for some land surface variables, in preparation for Met Office work on land surface DA with JEDI. The names I am planning to add are as follows:

1. surface_snow_density
2. mass_content_of_water_in_top_soil_layer
3. volume_fraction_of_liquid_water_in_soil_at_wilting_point
4. volume_fraction_of_liquid_water_in_soil_at_critical_point
5. volume_fraction_of_liquid_water_in_soil_at_saturation
6. urban_area_fraction
7. land_ice_area_fraction

I have used CF Standard Names (cfconventions.org) where possible, and where nothing suitable exists I have based the proposed CCPP name on the CCPP or CF name of a related or similar variable.

I think it would make sense to add a new sub-section for these surface variables - any views on whether this is for all surface variables (i.e. land, sea, sea-ice), or just restricted to land surface?

I will open a PR for these additions and link it to this issue - this is just a heads-up of the planned additions

[SamanthaPullen]

Draft PR #60 opened to address this issue

[ss421]

On the land_ice_area_fraction, there is some related discussion in #57. Ive added some notes there.

[mkavulich]

Resolved by #60