Merge pull request #3095 from jwpeterson/dirac_set

Miscellaneous DiracKernel improvements
Adding ability to specify a unique ID for a Dirac point.

This can be used to speed up the performance of DiracKernel::addPoint(p)
by avoiding the PointLocator step on subsequent additions of the same point.

Refs #2364.
Adding a test to demonstrate the Dirac point caching.

Refs #2364.
Adding an expect_err test that demonstrates the error handling when
you try to cache a point with a "unique" ID that's already taken.

Refs #2364.