Numpy usage error in `perform_blurring.perform_convolution`

There is an error in the `perform_convolution` function the python module
[`perform_blurring`](https://github.com/gpeyre/numerical-tours/blob/master/python/nt_toolbox/perform_blurring.py).

On line 218, the array `d` should be converted to an integer type as its used for indexing. 
Suggested fix:
```python
d = np.floor((p-1)/2.).astype(int)
```

Thank you in advance!