Issue ecmwf#470, allowing NumPy types in the sel() method.

- Added support for NumPy scalars by checking if the item has an 'item' method and calling it when applicable.
- Converted non-scalar NumPy arrays to lists to ensure consistent handling in the selection process.

src/earthkit/data/core/index.py

@@ -82,13 +82,22 @@ def __call__(self, x):
                 self.actions[k] = InSlice(v)
                 continue
 
+            if hasattr(v, 'item') and callable(v.item):
+                if not hasattr(v, 'shape') or not v.shape:
+                    v = v.item()  
+                else:
+                    v = v.tolist() 
+
             if not isinstance(v, (list, tuple, set)):
                 v = [v]
-
-            v = set(v)
-
+
+            v = [item.item() if hasattr(item, 'item') and 
+                callable(item.item) and (not hasattr(item, 'shape') 
+                or not item.shape) else item for item in v]
+
+            v = set(v) 
             self.actions[k] = InList(v)
-
+                  
     def match_element(self, element):
         metadata = self.remapping(element.metadata)
         return all(v(metadata(k, default=None)) for k, v in self.actions.items())