Only compute off-diagonal Jacobian entries if variable has DOFs.

This patch does the proper checking at the
ComputeDiracThread::onElement() level rather than the
DiracKernel::computeOffDiagJacobian() level, and undoes the quick fix
I committed earlier.

Refs #668.
Fix misc. warnings

Refs #1777.
Merge pull request #3444 from jwpeterson/fixup_dirac_offdiag

Only compute off-diagonal Jacobian entries if variable has DOFs.
Merge pull request #3445 from jwpeterson/fix_warnings

Fix misc. warnings
Fix sign error in the first spline derivative (#3426)
Merge pull request #3453 from dschwen/splinebug_3426

Fix sign error in the first spline derivative
Delete unused variable lines as requested by Cody.

Refs #1777.
Fixing an efficiency bug where the PointLocator was always rebuilt.

AKA fixing my brain damage.

Refs #2364.
Merge pull request #3455 from jwpeterson/debug_pl

Fixing an efficiency bug where the PointLocator was always rebuilt.
Repaired Debug block; added a test
(refs #3458)
Merge pull request #3460 from aeslaughter/debug-3458

Repaired Debug block; added a test