Add int64 as accepted data type

When using data coming from numpy `int64` ndarrays, the `bic` function crashes because of this (in `bic.py`):

```
if not isinstance(i, (int, float)):
```

This could be changed to:

```
if not isinstance(i, (int, float, np.int64)):
```

instead. I also guess this applies to the other functions of this package.