basic implementation of 1D *spectral* slices (WCS still has trouble with
non-spectral 1D slices)
fix a call to _new_cube_with and add a .value property to return a
unitless copy of self
implement @ChrisBeaumont's comments
REMOVE np.asarray: it does bad things to Quantities!
whitespace
asarray for value.ndim!  THAT's how you do it... riiight.
Merge pull request #157 from keflavich/slice_1d

Allow 1D Spectral Slices