Fix intermittent bug due to missing scotch graphinit.

Issue/feature addressed

Intermittent segmentation faults were being observed, particularly with recent versions of Scotch (7.0.3).

Proposed solution

Correctly initialise the scotch graph being used by the substructuring algorithm.

Implementation

Added missing call to SCOTCH_graphInit in the SubstructuredGraph class setup routine.

Tests

None needed, as this is tested by the existing test suite.

Checklist

• Functions and classes, or changes to them, are documented.
• User guide/documentation is updated.
• Changelog is updated.
• Suitable tests added for new functionality.
• Contributed code is correctly formatted. (See the contributing guidelines).
• License added to any new files.
• No extraneous files have been added (e.g. compiler output or test data files).