Fix ThirdParty scotch include counld not be found

Issue/feature addressed

_If the Scotch is not installed in the system globally, Nektar++ will build ThirdParty Scotch. But the CMake will report that SCOTCH_INCLUDE_DIR is set to SCOTCH_INCLUDE_DIRS-NOTFOUND. This issue only appears in feature/redesign

Proposed solution

Add SET(SCOTCH_INCLUDE_DIR ${TPDIST}/include)

Implementation

A more detailed description of the changes. Please include any details necessary for reviewers to understand your work, and point out particular points would like them to pay particular attention to.

Tests

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Notes

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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).