Be consistent on Tag and Geometry retrieval for OSMContribution

### Problem Description

To extract `OSHDBTags` from an `OSMContribution` one needs to extract the `OSMEntity` first like `contribution.getEntityBefore().getTags()`.

The extraction of the `Geometry` from an `OSMContribution` is located directly with the `OSMContribution` e.g. `contribution.getGeometryUnclippedBefore()`.

This seems incoherent and makes client code less readable.

### Expected Solution

Move `Geometry`-retrieving methods from `OSMContribution` to `OSMEntity`.

### Alternative Solutions

Move `OSHDBTags` retrieving methods to `OSMContribution`.