VectorPostprocessor System

This is the initial addition of the VectorPostprocessor system.  This system allows you to create Postprocessors who's output is one or more vectors.  The vectors are named and show up as columns in a CSV file.

The initial target application for this capability is to sample a solution along a line.  So far, all that implemented is a really dumb ConstantVectorPostprocessor that allows you to specify the values of one output vector.

I will be adding several more Postprocessors and tests to exercise all of the different interfaces I'm adding here but I wanted to get this up so that others can comment on it.  I'll add more commits after this one as I add concrete VectorPostprocessors... but as soon as someone wants they can go ahead and merge this one... and then I'll add more VectorPostprocessors using other tickets and PRs...

The big issue that is still left to figure out completely is how to output these buggers.  Right now they only come out with the CSV writer - and only then if you set `output_vector_postprocessors = true`.  If that's the case then the CSV writer will write one file per VectorPostprocessor with all of the vectors from that VectorPostprocessor in it.  I foresee MANY enhancements / modifications to that idea....

closes #3087