Interoperability testing for unstructured grids

The new resqpy unstructured grid classes have been unit tested. However, no inter-operability tests have yet been carried out reading datasets generated elsewhere, or transferring resqpy datasets with unstructured grids to non-resqpy software. Help with this would be appreciated.

Start by locating the unstructured grid classes and their existing unit tests, then identify external datasets and non-resqpy software suitable for interoperability checks. Done means tests demonstrate reading externally generated unstructured-grid datasets and transferring resqpy datasets to other software.

Written by the indexing model from the issue text.