- moves any nullptr to the end of the list where they can be
  eliminated in a second step with resize()

- when combining lists in processor order this simplifies code and
  reduces memory overhead.

  Write this:
    ----
    labelList collected;

    const globalIndex sizing(input.size());
    sizing.gather(input, collected);
    ----

  OR

    ----
    labelList collected;
    globalIndex::gatherOp(input, collected);
    ----

  Instead of this:

    ----
    labelList collected;

    List<labelList> scratch(Pstream::nProcs());
    scratch[Pstream::myProcNo()] = input;
    Pstream::gatherList(scratch);

    if (Pstream::master())
    {
        collected = ListListOps::combine<labelList>
        (
            scratch,
            accessOp<labelList>()
        );
    }
    scratch.clear();
    ----

- remove unused and deprecated emptyList() casting function.

  This function is disllowed by many modern compilers and is no longer
  used within OpenFOAM - was deprecated 2018-07.

- use forwarding tmp factory methods, auto types

- use forwarding tmp factory methods, auto types

- use forwarding tmp factory methods, auto types

- use forwarding tmp factory methods, auto types

- use forwarding tmp factory methods, auto types

- use forwarding tmp factory methods

- use local 'prefix' variable for easier override and more consistency

- can still test as a bool or use to 'peek' at the content.

- make adjustment of the DebugSwitches optional.
  Enable (the old behaviour) with -debug.

- only backup/modify controlDict if it contains a DebugSwitches entry

- add -no-debug option to suppress modification entirely

- write values of processor boundaries only, for diagnostic purposes.

- parallel only, and non-legacy format only

- The dummy accessOp can be useful outside of ListListOps.
- New emptyOp for using as a filter predicate (for example).

- this protected method was previously used directly for the list
  output and had the check for 2 or more elements in it. Now simply
  test the List content and handle the output preference separately.

- generates dictionary-like content, but without a separator between
  keyword and list content.