Provide C++20-style operator| to filter iterator ranges.

[bangerth]

In the same spirit as provided by the C++20 range adaptors feature, this little function makes it possible to "filter" ranges over the active cells of Triangulation and DoFHandler objects.

This allows to replace:

  DoFHandler<dim> dof_handler;
  ...
  for (const auto &cell : dof_handler.active_cell_iterators())
    {
      if (cell->is_locally_owned())
        {
          fe_values.reinit (cell);
          ...do the local integration on 'cell'...;
        }
    }

by:

  DoFHandler<dim> dof_handler;
  ...
  const auto filtered_iterators_range =
    filter_iterators();
  for (const auto &cell :
          dof_handler.active_cell_iterators() | IteratorFilters::LocallyOwnedCell())
    {
      fe_values.reinit (cell);
      ...do the local integration on 'cell'...;
    }

Here, the operator| is to be interpreted in the same way as is done in the range adaptors feature that is part of C++20. It has
the same meaning as the | symbol on the command line: It takes what is on its left as its inputs, and filters and transforms to produce some output. In the example above, it "filters" all of the active cell iterators and removes those that do not satisfy the predicate -- that is, it produces a range of iterators that only contains those cells that are both active and locally owned.

I've wanted to do this for a long time because the syntax of range adaptors (of which filters are a special case) is so elegant. It turns out that if one looks at what we already have, it was all there: We just had it in a free function filter_iterators(), rather than an operator|().

The last commit here converts a few random places to see whether this all actually works :-)

/rebuild

[masterleinad]

Looks good to me!

[masterleinad]

You need to resolve some merge conflicts.

/rebuild

[bangerth]

Rebased. Let's see whether this works.

[bangerth]

Yay. I really like this syntax :-)