tractionDisplacementFvPatchVectorField: changed applied pressure to be +ve from the outside

Resolves bug-report http://www.openfoam.org/mantisbt/view.php?id=763

applications/solvers/stressAnalysis/solidDisplacementFoam/tractionDisplacement/tractionDisplacementFvPatchVectorField.C

@@ -2,7 +2,7 @@
   =========                 |
   \\      /  F ield         | OpenFOAM: The Open Source CFD Toolbox
    \\    /   O peration     |
-    \\  /    A nd           | Copyright (C) 2011 OpenFOAM Foundation
+    \\  /    A nd           | Copyright (C) 2011-2013 OpenFOAM Foundation
      \\/     M anipulation  |
 -------------------------------------------------------------------------------
 License
@@ -181,7 +181,7 @@ void tractionDisplacementFvPatchVectorField::updateCoeffs()
 
     gradient() =
     (
-        (traction_ + pressure_*n)/rho
+        (traction_ - pressure_*n)/rho
       + twoMuLambda*fvPatchField<vector>::snGrad() - (n & sigmaD)
     )/twoMuLambda;