MaterialStdVectorAux added.  Fixes #3590

This allows extraction of a component of a std::vector<Real>
from a Material.
Aha, a mooseError is not good here.  At the start of the simulation, the std::vector may not have been resized appropriately, and this "if" statement is to catch that.  Once the simulation gets going, everything is OK.  If i put mooseError then all my simulations fail, and if i don't have the "if" i get a segfault.