Add a load method to the Dat class

Adds a `load` method to the `Dat` class to read in values from a .npy file and store them in `self._data`.
In which case the better option is probably:

```python
for d, d_from_file in zip(self, np.load(filename)):
    d.data[:] = d_from_file[:]
```

Iterating over a mixeddat yields the individual dats in it, iterating over a dat just yields the dat itself.