Power of dimension are not casted to int when possible

The power of dimension object could be (should be) casted back to int when possible. Should be discussed : 
 - what are the advantages to casting back to int
 - what is associated speed loss 

It is not the case for now : 

```python
from physipy import m, K

a = (m**2)**0.5
print(a.dimension)
# --> L**1.0
```