Switch default serial LU solver when using 64-bit indices.

This follows a bug fix in PETSc.
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Improve smoothed aggregation AMG test and update ChangeLog.
Merge branch 'master' into garth/fix-issue-430-samg

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Merge branch 'garth/fix-issue-430-samg' into next
Merge branch 'master' into next

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Merge branch 'garth/switch-default-lu-solver-64-bit' into next