ENH: simplify construction of zero-sized Clouds

- use Foam::zero as a dispatch tag

FIX: return moleculeCloud::constProps() List by reference not copy

STYLE: range-for when iterating cloud parcels

STYLE: more consistent typedefs / declarations for Clouds

applications/utilities/preProcessing/mapFieldsPar/mapLagrangian.C

@@ -129,6 +129,7 @@ void mapLagrangian(const meshToMesh& interp)
                 cloudDir,
                 false
             );
+
             Info<< "    read " << sourceParcels.size()
                 << " parcels from source mesh." << endl;
 
@@ -136,8 +137,8 @@ void mapLagrangian(const meshToMesh& interp)
             passiveParticleCloud targetParcels
             (
                 meshTarget,
-                cloudDir,
-                IDLList<passiveParticle>()
+                Foam::zero{},
+                cloudDir
             );
 
             passiveParticle::trackingData td(targetParcels);