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.
Adding Dirac point reverse cache id lookup as requested by @WilkAndy

Refs #2364.
Update the CachingPointSource test to look up the current point's ID.

Also fixed BCs and renamed the output and gold files to avoid clashing
with an expect_err test in the same directory that used the same
filename.

Refs #2364.
Use DiracKernel caching and currentPointCachedID()
in RichardsBorehole and RichardsPolyLineSink.

Refs #2364.
Handle caching when Elem containing Point is not active.

The other two cases of interest,
* (active && !contains_point)
* (!active && !contains_point)

currently fall back to PointLocator searches, but I have
some ideas for how these could be optimized.  Still need
to add a test case for the refinement case.

Refs #2364.
Adding test of uniform refinement with Dirac point caching.

Refs #2364.
Adding test of adaptive refinement with Dirac point caching.

Refs #2364.