Fixes for issues #22 and #23 (#24)

* Fix for AxisError with numpy 1.18

FFN fails with
`numpy.AxisError: axis 4 is out of bounds for array of dimension 4`

when numpy 1.18 is used.

See the 1.18 note for numpy.expand_dims at
https://numpy.org/doc/stable/reference/generated/numpy.expand_dims.html
```
axis int or tuple of ints
Position in the expanded axes where the new axis (or axes) is placed.

Deprecated since version 1.13.0: Passing an axis where axis > a.ndim will be treated as axis == a.ndim, and passing axis < -a.ndim - 1 will be treated as axis == 0. This behavior is deprecated.

Changed in version 1.18.0: A tuple of axes is now supported. Out of range axes as described above are now forbidden and raise an AxisError.
```

* Fix import error

Training fails since Optional was not imported

ffn/training/inputs.py

@@ -152,7 +152,7 @@ def _load_from_numpylike(coord, volname):
     if data.ndim == 4:
       data = np.rollaxis(data, 0, start=4)
     else:
-      data = np.expand_dims(data, 4)
+      data = np.expand_dims(data, data.ndim)
 
     # Add flat batch dim and return.
     data = np.expand_dims(data, 0)

ffn/utils/bounding_box.py

@@ -31,6 +31,7 @@
 from . import bounding_box_pb2
 from . import geom_utils
 
+from typing import Optional
 
 class BoundingBox(object):
   """BoundingBox built on Numpy, interoperable with bounding_box_pb2."""