Add World Coord Value of Sliced out Dimension of NDCube to Extra_coords.

Example:
Let the ```NDCube``` have three dimensions, space, space, wavelength.
Slice out the wavelength dimension: 
```python
>>> new_cube = NDCube[:, :, 0]
```
Value of wavelength at that sliced pixel should be in extra_coords:
```python
>>> new_cube.extra_coords
{"wavelength": {"axis": None, "value": 1 nm}}
```