ENH: robuster handling of system or central installation (issue #290)

- Follow similar pattern as per boost/CGAL with the '*-none' to disable,
  and '*-system' for system installations.

  Also support central non-ThirdParty installations by specifying the
  version as "*-system", but also providing the fully qualified
  *_ARCH_PATH too.

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ENH: Verify label and scalar sizes used in the metis header.

- ensure they match with expected values, which can be obtained via
  the OpenFOAM definitions for WM_LABEL_SIZE and WM_PRECISION_OPTION