Allow dropped cameras and use COLMAP export format

Something like:
```python
from typing import Dict, List, NamedTuple

class SfmDataPython(NamedTuple)
cameras: Dict[int,SfmCamera]
tracks: List[SfmTrack]
```

Can COLMAP format support non-existing indices (non-contiguous indices)?

- If so, replace BAL output with COLMAP output format