Compute only AuxKernels of the correct type

I'm reviving the patch from @andrsd to fix the AuxKernel problem.  This is mostly working but failing on about a half dozen tests between moose_test and the modules due to initial conditions for certain tests appearing now where they weren't before.  It makes sense that they are there but I need to go back and see why they weren't before.

@andrsd - I'm not sure about the "need_materials" logic in ```AuxiliarySystem::computeElementalVars()```  How can you know for certain that you won't need materials for any AuxKernel based just on the execute flag?

Other things to review would be how I've chosen to handle the legacy flag.  Individual apps can choose the global default, but this can be overridden per problem.  For now, I set it on just the combined module but the individual modules don't have it set so we'll see several failures.  I plan to update those individual tests rather than use the legacy logic.

Stay tuned...