line source example

We've been messing around with embedding 2 dimensional surfaces inside of 3 dimensional domains... BUT - what about embedding 1D surfaces into 3D domains?  It works!  And what it lets us do is _line sources_... something that has been highly sought after by several MOOSE users (like @WilkAndy ).

This test shows an example of embedding a 1D line in the middle of a 3D domain.  I use it to inject a heat source term into the middle of the 3D domain and then I use a Postprocessor to verify that I'm getting the correct flux....

 #3990