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

[DanRyanIrish]

Example:
Let the NDCube have three dimensions, space, space, wavelength.
Slice out the wavelength dimension:

>>> new_cube = NDCube[:, :, 0]

Value of wavelength at that sliced pixel should be in extra_coords:

>>> new_cube.extra_coords
{"wavelength": {"axis": None, "value": 1 nm}}

[kakirastern]

I will attempt to add this feature soon.

[kakirastern]

PR opened for this issue.

[DanRyanIrish]

This issue has been incorporated into #284