ENH: Add NestedHybridGrid class (#464)

Author: tnatt

src/fmu/tools/nestedhybridgrid/__init__.py

@@ -1,4 +1,5 @@
 from .nestedhybrid import (
+    NestedHybridGrid,
     create_nested_hybrid_grid,
     nnc_to_flowsimulator_input,
     nnc_to_gridproperty,
@@ -8,4 +9,5 @@
     "create_nested_hybrid_grid",
     "nnc_to_flowsimulator_input",
     "nnc_to_gridproperty",
+    "NestedHybridGrid",
 ]