ENH: cleanup of ListOps, ListListOps. Adjustments to List, PackedList.

- relocated ListAppendEqOp and ListUniqueEqOp to ListOps::appendEqOp
  and ListOps::UniqueEqOp, respectively for better code isolation and
  documentation of purpose.

- relocated setValues to ListOps::setValue() with many more
  alternative selectors possible

- relocated createWithValues to ListOps::createWithValue
  for better code isolation. The default initialization value is itself
  now a default parameter, which allow for less typing.

  Negative indices in the locations to set are now silently ignored,
  which makes it possible to use an oldToNew mapping that includes
  negative indices.

- additional ListOps::createWithValue taking a single position to set,
  available both in copy assign and move assign versions.
  Since a negative index is ignored, it is possible to combine with
  the output of List::find() etc.

STYLE: changes for PackedList

- code simplication in the PackedList iterators, including dropping
  the unused operator() on iterators, which is not available in plain
  list versions either.

- improved sizing for PackedBoolList creation from a labelUList.

ENH: additional List constructors, for handling single element list.

- can assist in reducing constructor ambiguity, but can also helps
  memory optimization when creating a single element list.
  For example,

    labelListList labels(one(), identity(mesh.nFaces()));