- Introduce `RangeDataInserterInterface` as common interface for all range data inserters
- Introduce a minimal `GridInterface` as a base for a common interface for 2D and 3D grids
- Rename `RangeDataInserter2D` to `ProbabilityGridRangeDataInserter2D`
- Move grid generation logic from `Submap2D` to `ActiveSubmaps2D`
- Update proto and configuration structure to mirror the code structure - backwards compatibility is maintained
- Step towards [RFC 0019](https://github.com/googlecartographer/rfcs/blob/master/text/0019-probability-grid-and-submap2d-restructuring.md)