COMP: fix sloppy (and now ambiguous) use of PtrList::set()

- constructs such as the following will no longer worked, but that is
  also a good thing.

     ptrlist.set(i, scalarField(nFaces, Zero));

  this called set(.., const tmp<scalarField>&), which meant under
  the hood:

     - create local temporary const scalarField&
     - wrap as const tmp&
     - use tmp::ptr(), to clone the const-ref

  This implies an additional allocation (for the const scalarField&)
  which is immediately discarded. Doubtful that compiler optimization
  would do anything.

src/optimisation/adjointOptimisation/adjoint/optimisation/updateMethod/LBFGS/LBFGS.C

@@ -58,8 +58,8 @@ void Foam::LBFGS::allocateMatrices()
     label nVars(activeDesignVars_.size());
     for (label i = 0; i < nPrevSteps_; i++)
     {
-        y_.set(i, scalarField(nVars, Zero));
-        s_.set(i, scalarField(nVars, Zero));
+        y_.set(i, new scalarField(nVars, Zero));
+        s_.set(i, new scalarField(nVars, Zero));
     }
 }
 
@@ -120,8 +120,8 @@ void Foam::LBFGS::LBFGSUpdate()
     label nSteps(min(counter_, nPrevSteps_));
     label nLast(nSteps - 1);
     scalarField q(objectiveDerivatives_, activeDesignVars_);
-    scalarField a(nSteps, 0.);
-    scalarField r(nSteps, 0.);
+    scalarField a(nSteps, Zero);
+    scalarField r(nSteps, Zero);
     for (label i = nLast; i > -1; --i)
     {
         r[i] = 1./globalSum(y_[i]*s_[i]);