fix: starting unit id @ 1 means basement =0

If no basement included in stratigraphic column, then

LoopStructural/modelling/input/process_data.py

@@ -164,7 +164,7 @@ def stratigraphic_column(self):
         stratigraphic_column = {}
         # add stratigraphy into the column
         if self.stratigraphy:
-            unit_id = 0
+            unit_id = 1
             val = self._stratigraphic_value()
             for name, sg in self._stratigraphic_order:
                 stratigraphic_column[name] = {}