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.