Mark OLESEN authored Nov 25, 2022 and Andrew Heather committed Dec 01, 2022

- make fileHandler deletion mechanism more
  transparent by providing a nullptr signature. A nullptr parameter
  is already being used in the argList destructor for shutdown, but that
  relied on an implicit conversion to autoPtr to trigger things.

- improved handling of file handler replacement.

  Previously had a very basic check on old vs new handlers using their
  type() values (string comparison!!), which would unfortunately
  prevent proper swapping of the contents.
  Check the actual pointers instead.

  As part of the change, treat any empty autoPtr as no-op instead of as
  deletion (which is handled explicitly as nullptr instead).

  In addition to making the internal logic simpler, it means that the
  current file handler always changes to a valid state without
  inadvertently removing everything and falling back to creating a new
  default handler (again).

  This handling of no-ops also simplifies call code. For example,

  <code>
      autoPtr<fileHandler> oldHandler;
      autoPtr<fileHandler> writeHandler;
      word handlerName;

      if (arg.readIfPresent("writeHandler", handlerName))
      {
          writeHandler = fileOperation::New(handlerName);
      }

      oldHandler = fileHandler(std::move(writeHandler));

      ... do something

      writeHandler = fileHandler(std::move(oldHandler));
  </code>

  If the "writeHandler" is not specified, each call is a no-op.
  If it is specified, the handlers are swapped out each time.

- the management of the fileHandler communicators is now encapsulated
  privately (managedComm_) with the final layer being responsible for
  cleaning up after itself. This makes delegation/inheritance clearer
  and avoids the risk of freeing an MPI communicator twice.

STYLE: uniformFile static check relocated to fileOperation layer