I'm not convinced we need/want this `Interpolator` class.

Within the scope of this PR it'd be better to ditch the extra indirection and complexity because there's only a single subclass.

If you want to look further ahead, we've been talking about using classes such as `Linear` for both interpolation and re-gridding. But some of the algorithms (e.g. your point-in-cell algorithm) won't have support for interpolation, which seems at odds with having a base class which defines an `interpolator()` method.