testing importError with new CI

[shimwell]

Testing of import errors would ideally be implemented into the test suite.

We don't currently test the following lines

    except (ImportError, ModuleNotFoundError):
        msg = "Conversion to VTK requested," \
            "but the Python VTK module is not installed."
        raise ImportError(msg)

In this particular case this vtk package is needed for the neutronics but not needed for geometry construction.

So we could have another CI that performs geometry only tests and check these.

The new CI could be based on a Docker image that does not include any neutronics dependancies