Reaction force error+design improvement for FESystem

[yangbai90]

The gpProj vector in the Elmt class is incorrectly assembled; it should be node independent. An additional function to save the global residual array should be implemented for the reaction force.

For instance,

[elmta]
  [test]
    type=mechanics
    save_residual=true
   [end]
[end]

The projection should still be responsible for projecting the quantities from gauss point to nodal points.

Tasks for this issue:

• [ ] fix gpProj assemble error
• [ ] functions for saving and accessing global/local residual vector
• [ ] re-design the sub-elmt class