Documenting Assumption road improvement

@XaviEspinet As we're moving into the selection of road improvements, can you confirm the following assumptions:

### Intervention types
- **Upgrade of surface treatment**
Means paving a previously unpaved road. Can be applied to all segments with:
- `SURF_TYPE=Unpaved`
- **Upgrade of gravel roads**
Restoring paved roads to a good condition. Can only be applied to segments with:
- `SURF_TYPE=Paved`
- `AVG_COND=Poor` or `AVG_COND=Fair`
- **Rehabilitation of earth roads**
Restoring unpaved roads to a good condition. Can only be be applied to segments with:
- `SURF_TYPE=Unpaved`
- `AVG_COND=Poor` or `AVG_COND=Fair`
- **Clean and repair bridges**
Restoring all the bridges along a road segment to their original condition.
- **Replacing culverts**
Replacing all the culverts along a road segment.

### Other assumptions

- the first three improvements are mutually exclusive. Only one can be applied to a given road segment.
- the last two improvements can be applied simultaneously and to any road with bridges and/or culverts

## Questions

- there are 114 road segments that don't have a Surface Type and Condition. Can we use a default surface type and condition in these cases? Unpaved and poor condition?