WIP: Implement PersistentGridView

This caches any accessed index lazily. The grid view only implements indices but no iteration because this is not needed for the intended use:

Create a persistent version of a basis that survives grid modifications in order to interpolate solutions to a refined grid. Notice that this approach is non-intrusive, i.e. the bases don't have to be adjusted, but simply work, on the PersistentGridView, as long as they only rely on the provided feature set.