Bohrium API: supply a way to check if an array needs to be flushed

Could be helpful for smart `__repr__`s:

```python
>>> a = bh107.random.rand(2, 2)
>>> b = bh107.random.rand(2, 2)
>>> c = a + b
>>> c  # results are not computed yet
BhArray([[- -],
         [- -]])
>>> bh107.flush()
>>> c
BhArray([[1 2],
         [3 4]])
```

I think this would be nice sugar to help people understand what's going on inside Bohrium.