[writer] with syntax for field insertion?

[dglaeser]

trafficstars

We could have with_point_field, with_cell_field with_meta_data functions in the writers to allow chaining such as:

GridFormat::VTUWriter{grid}
    .with_point_field("pfield", [&] (const auto& p) { ... })
    .with_cell_field("cfield", [&] (const auto& p) { ... })
    .write("filename");

[dglaeser]

Not super critical for a first release, removing the milestone...