FieldVectors are not comparable, but hashable.

This should make them more usable in the context of config files.

[subdomain] refactor part to tunnel correct iterators to outside

Pylicense update

dailywork-ftschindler

Allow any layer in periodic view

See https://gitlab.dune-project.org/dune-fem/dune-fem/issues/59.

* PeriodicGridView now accepts any layer, view or part
* adds PeriodicGridPart, accepts grid parts, is a grid part
* adds make_periodic_grid_part
* adds PeriodicGridLayer chooses the matching one
* adds make_periodic_grid_layer