ENH: issue warning or error if output surface is empty or near empty

If too few active map nodes, less than 4, a warning will be given when to_file() is called. In addition a new key error_if_near_empty is added, and if this is True, an error will be raised in stead.

src/xtgeo/surface/regular_surface.py

@@ -1097,6 +1097,7 @@ def to_file(
         fformat: Optional[str] = "irap_binary",
         pmd_dataunits: Optional[Tuple[int, int]] = (15, 10),
         engine: Optional[str] = "cxtgeo",
+        error_if_near_empty: bool = False,
     ):
         """Export a surface (map) to file.
 
@@ -1117,6 +1118,9 @@ def to_file(
                 but may be safer when reading memory streams and/or threading. Engine
                 is relevant for Irap binary, Irap ascii and zmap. This is mainly a
                 developer setting.
+            error_if_near_empty: Default is False. If True, raise a RuntimeError if
+                number of map nodes is less than 4. Otherwise, if False and number of
+                nodes are less than 4, a UserWarning will be given.
 
         Returns:
             ofile (pathlib.Path): The actual file instance, or None if io.Bytestream
@@ -1145,8 +1149,20 @@ def to_file(
         .. versionchanged:: 2.5 Added support for BytesIO
         .. versionchanged:: 2.13 Improved support for BytesIO
         .. versionchanged:: 2.14 Support for alias file name and return value
+        .. versionchanged:: 3.8 Add key ``error_if_near_empty``
         """
         logger.info("Export RegularSurface to file or memstream...")
+        if self.nactive is None or self.nactive < 4:
+            msg = (
+                f"Number of maps nodes are {self.nactive}. Exporting regular "
+                "surfaces with fewer than 4 nodes will not provide any "
+                "usable result. The map may also be not loaded if nodes are None."
+            )
+
+            if error_if_near_empty:
+                raise RuntimeError(msg)
+            warnings.warn(msg, UserWarning)
+
         mfile = FileWrapper(mfile, mode="wb", obj=self)
 
         if not mfile.memstream:

tests/test_surface/test_file_io.py

@@ -174,3 +174,21 @@ def test_read_cube(data):
     # Make sure it is properly constructed:
     result = {key: getattr(surf_from_cube, key) for key in cube_input}
     assert result == cube_input
+
+
+def test_surf_io_few_nodes(tmp_path, larger_surface):
+    """Test to_file() with to few active nodes shall warn or raise error."""
+
+    surf = RegularSurface(**larger_surface)
+
+    assert surf.nactive == 38
+
+    surf.values = np.ma.masked_where(surf.values > 0, surf.values)
+
+    assert surf.nactive == 3
+
+    with pytest.warns(UserWarning, match="surfaces with fewer than 4 nodes will not"):
+        surf.to_file(tmp_path / "anyfile1.gri")
+
+    with pytest.raises(RuntimeError, match="surfaces with fewer than 4 nodes will not"):
+        surf.to_file(tmp_path / "anyfile2.gri", error_if_near_empty=True)