Switch to BoxMarker in point_caching_adaptive_refinement test.

Also displacing the Dirac points off the element corners slightly, to
make this example more robust in parallel.

Refs #4442.
Fixed logic error resulting invalid postprocessor warning
(closes #4435)
Add VectorPostprocessors to get materials along a line closes #4462

This provides an abstract base class, LineMaterialSamplerBase, that
has the basic machinery for extracting the material properties for the
elements along a line, and two derived classes,
LineMaterialRealSampler and LineMaterialSymmTensorSampler, for
extracting data for specific material types.
Merge pull request #4460 from aeslaughter/warnings-4435

Fixed logic error resulting invalid postprocessor warning
Merge pull request #4463 from bwspenc/material_vectorpost

Add VectorPostprocessors to get materials along a line
Merge pull request #4452 from jwpeterson/dirac_adapt

Switch to BoxMarker in point_caching_adaptive_refinement test.