decomposePar: Added 'copyZero' option

Using

decomposePar -copyZero

The mesh is decomposed as usual but the '0' directory is recursively copied to
the 'processor.*' directories rather than decomposing the fields.  This is a
convenient option to handle cases where the initial field files are generic and
can be used for serial or parallel running.  See for example the
incompressible/simpleFoam/motorBike tutorial case.