Add `index` field to schema on the fly

This is a UX issue: the user should not care about how we index rows.

```python
class EducationItem(BaseModel):
    index: int
    year: Optional[List[str]]
    university: Optional[List[str]]
```
should become
```python
class EducationItem(BaseModel):
    year: Optional[List[str]]
    university: Optional[List[str]]
```
and the code should modify the model. 