In `markov.py` and `law.py` use of specific column names

in `amrkov.py`:
```python
length_max_secondary = length_data[0].LR_length_mm.max() * 1e-3 
```

Which is not general we should use 1st column without name. 

Idem in `law.py`
```python
x = pd.relative_distance_to_tip.tolist()
y = pd.LR_length_mm.tolist()
```