Merge branch 'icon_soil_init' into develop (PR #1298)

This merge adds the ability to use ICON soil variables in the init_atmosphere
core. Specifically, the init_atm_cases_gfs routine has been modified to
recognize and horizontally interpolate the fields

'SOILM001', 'SOILM002', 'SOILM006', 'SOILM018', 'SOILM054', 'SOILM162',
'SOILM486', 'SOILM999',

and

'SOILT001', 'SOILT002', 'SOILT006', 'SOILT018', 'SOILT054', 'SOILT162',
'SOILT486', 'SOILT999'.

* icon_soil_init:
  added capability for understanding ICON soil variables

Author: mgduda