Allow astrodendro to run on spectral_cubes

[keflavich]

Right now, it is not possible: one has to pass cube.filled_data[:] to astrodendro. That's maybe OK, but we can possibly make it 1 step easier? Even filled_data isn't working:

Traceback (most recent call last):
  File "<ipython-input-8-bd6d23720165>", line 1, in <module>
    dend = astrodendro.Dendrogram.compute(cube.filled_data[:], min_delta=2*rms*u.K, min_value=2*rms*u.K, min_npix=10)
  File "/Users/adam/repos/dendro-core/astrodendro/dendrogram.py", line 231, in compute
    leaf = Structure(coord, data_value, idx=idx, dendrogram=self)
  File "/Users/adam/repos/dendro-core/astrodendro/structure.py", line 78, in __init__
    self._values = [x for x in values]
  File "/Users/adam/repos/astropy/astropy/units/quantity.py", line 860, in __iter__
    .format(cls=self.__class__.__name__))
TypeError: 'Quantity' object with a scalar value is not iterable