ENH: simplify assign zero for Dimensioned and Geometric fields (#3402)

- regular or forced assignment from `zero` does not need dimension
  checking:

    Old:  U = dimensionedVector(U.dimensions(), Zero);
    New:  U = Zero;

  this eliminates a fair bit of clutter and simplifies new coding
  without losing general dimension checking capabilities.

src/functionObjects/forces/forceCoeffs/forceCoeffs.C

@@ -122,8 +122,8 @@ void Foam::functionObjects::forceCoeffs::reset()
     Cf_.reset();
     Cm_.reset();
 
-    forceCoeff() == dimensionedVector(dimless, Zero);
-    momentCoeff() == dimensionedVector(dimless, Zero);
+    forceCoeff() == Zero;
+    momentCoeff() == Zero;
 }
 
 

src/functionObjects/forces/forces/forces.C

@@ -200,8 +200,8 @@ void Foam::functionObjects::forces::reset()
 
     if (porosity_)
     {
-        force == dimensionedVector(force.dimensions(), Zero);
-        moment == dimensionedVector(moment.dimensions(), Zero);
+        force == Zero;
+        moment == Zero;
     }
     else
     {

src/functionObjects/solvers/scalarTransport/scalarTransport.C

@@ -195,7 +195,7 @@ Foam::functionObjects::scalarTransport::scalarTransport
 
     if (resetOnStartUp_)
     {
-        s == dimensionedScalar(dimless, Zero);
+        s == Zero;
     }
 }