ISCN3 variable table documentation

[ktoddbrown]

Add documentation to the variable table from ISCN3. Add a description and any common units to the master variable table. Including:

• al_dith
• al_other
• al_ox
• base_sum
• bs
• bs_sum
• bulk_density_other
• bulk_density_sample
• bulk_density_total
• bulk_density_whole
• ca_al
• ca_ext
• cec_sum
• data_collection
• dataset_type
• ecec
• ecoregion
• fe_dith
• fe_other
• fe_ox
• h_ext
• horizon
• horizon_designation
• horizon_designation_other
• k_ext
• landuse
• latitude
• layer_ISCN_soc_count
• layer_bottom
• layer_carbon_count
• layer_count
• layer_soc_count
• locator_alias
• locator_parent_alias
• longitude
• mg_ext
• mn_dith
• mn_other
• mn_ox
• na_ext
• p_bray
• p_meh
• p_other
• p_ox
• profile_ISCN_soc_count
• profile_bottom
• profile_count
• profile_soc
• profile_soc_carbon_flag
• profile_soc_count
• profile_soc_depth
• profile_soc_spatial_flag
• profile_top
• profile_zero_ref
• site_ISCN_soc_count
• site_count
• site_soc_count
• zn_ext