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`.
I don't think so. In the non-mixed case `self.data` is of type `numpy.ndarray` so just using
```
for d, d_from_file in zip(self.data, np.load(filename)):
    d[:] = d_from_file[:]
```
gives:
```
_________________________________________________________________________ TestDat.test_dat_save_and_load[sequential-greedy] _________________________________________________________________________
test_dats.py:133: in test_dat_save_and_load
>       d2.load('output')
/usr/local/lib/python2.7/dist-packages/PyOP2-0.10.0_410_gca88e7d_dirty-py2.7-linux-x86_64.egg/pyop2/base.py:1763: in load
>           d[:] = d_from_file[:]
E           IndexError: invalid index to scalar variable.
```