FIX: only drop existing variables from dataset

ROMSOC_tools/make_ROMSOC.py

@@ -66,7 +66,9 @@ def get_ROMSOC_ds(ROMS_grd_file, COSMO_grd_file, transition_width):
               cos_grd['rotated_pole'].attrs['grid_north_pole_latitude'])
         cos_map = dp_map(NP)
 
-    romsoc_ds = romsoc_ds.drop(('height_2m', 'height_10m'))
+    drop_vars = [v for v in ('height_2m', 'height_10m') if v in romsoc_ds]
+    if drop_vars:
+        romsoc_ds = romsoc_ds.drop(drop_vars)
 
     # Add velocity directions
     # =======================